The Format of the Social Security numbers was historically broken into 3 parts.

Area
3 Digits, The Area was assigned to States, US territories, or special use areas such as 'enumeration at entry' or for a brief period of the time Rail Road use employees (ended July 1, 1963). Of note is that this is from the STATE or area that person Applies, not necessarily where they are born. From 1973 - July 14, 2011 the Area was assigned by looking at the zip code shown on the mailing address that was used for the application. Before 1973 the The number was assigned by a field office. Area number of 000 is never assigned. For religious reasons, 666 was never allocated for assigment by SSA. As of 2009, area 772 was considered the highest issued area number and at the time was used to help identify fraudulent numbers.

Group
2 Digits, These were assigned in a sequency and it is from this number that approximate age of a SSN can be determined. This is the order a number was assigned in a particular AREA, and do not have any geographic designation. Historically SSA published a list of of the last used GROUP number by region and this is how age of the group was determined. This report was called "Highest Group Issued" or highgroup.txt (see below) and until SSN assignment was moved to Random, this report was key to determining fraudulent numbers. If a number was not yet used by a region it was fraudulent. If someone claims a SSN that was allocated before the person was born, then it is most probably fraudulent. Group 00 is never used.
Order of issuance was:
Odd 01-09
Even 10-98
Even 02-08
Odd 11-99

Block
5 Digits, NumChk reports on approximately 100,000 of these numbers or blocks and this is how the content is organized.

Serial
4 Digits, this is the serial number portion of a SSN number. Because the first 5 digits of older SSN numbers can be determined by State/Region and approximate date of issue, the Serial number is the only real 'secret' part of the number. Unfortunately, many publish this part of the number to confirm the number is yours, and this is the hardest part of the number to determine in most cases if someone were to brute force attack (try to guess) your SSN, so by people publishing this they are doing you a disservice - and further exposing you to identify theft. Even if left to brute force, at only 4 digits, this leaves 10k combinations that is still trivial to determine. Serial 0000 is never assigned.

Serial numbers are normally assigned in chronological order within each area and group number as the applications are processed. Before 1965, serial numbers may have been assigned in a strange order. Reports are 2000 and 7000 series numbers were assigned out of order. From 1965 - 2011 the serial numbers were assigned in strictly increasing order with each area and group combination.

Since 2011 and Randomization - all new numbers including the Serial number are random from valid number allocations and no longer to determine the order, and thus approximate age of issuance.

An Example of the highgroup.txt file historically published by the SSA.
        HIGHEST GROUP ISSUED AS OF 7/01/08


        Anything with an asterisk (*) is a change effective 7/01/08.
        This list shows the SSN area and group numbers that are in
        the process of being issued as of the date at the top of this page.
        
        NOTE:  INDICATES GROUP CHANGE SINCE LAST MONTH.
        001 06  002 06	003 06	004 08	005 08	006 08
        007 08	008 92	009 90  010 92  011 92  012 92*
        013 90	014 90	015 90	016 90	017 90	018 90
        019 90	020 90	021 90	022 90	023 90	024 90
        025 90	026 90	027 90	028 90 	029 90	030 90
        031 90	032 90	033 90	034 90	035 72	036 72
        037 72  038 72	039 72	040 13	041 13  042 13
        043 11	044 11	045 11	046 11	047 11	048 11 
        049 11	050 98	051 98	052 98	053 98	054 98
        055 98	056 98	057 98	058 98	059 98* 060 98* 
        061 98* 062 98* 063 96  064 96  065 96	066 96
        067 96	068 96	069 96	070 96	071 96	072 96
        073 96	074 96	075 96	076 96	077 96	078 96
        079 96	080 96	081 96	082 96	083 96	084 96
        085 96	086 96	087 96	088 96	089 96	090 96
        091 96	092 96	093 96	094 96	095 96	096 96
        097 96	098 96	099 96	100 96	101 96	102 96
        103 96	104 96	105 96	106 96	107 96	108 96
        109 96	110 96	111 96	112 96	113 96  114 96 
        115 96  116 96  117 96  118 96  119 96	120 96
        121 96	122 96	123 96	124 96	125 96	126 96 
        127 96  128 96	129 96	130 96	131 96	132 96
        133 96	134 96	135 19	136 19	137 19	138 19
        139 19	140 19	141 19	142 19  143 19	144 19 
        145 19 	146 19	147 19	148 19	149 19	150 19
        151 19	152 19	153 19	154 19  155 19	156 19
        157 19	158 19*	159 84	160 84	161 84	162 84
        163 84	164 84	165 84	166 84	167 84	168 84
        169 84	170 84	171 84	172 84	173 84	174 84
        175 84	176 84	177 84	178 84	179 84	180 84 
        181 84 	182 84	183 84	184 84	185 84	186 84
        187 84	188 84	189 84	190 84	191 84	192 84
        193 84	194 84*	195 84* 196 82	197 82	198 82
        199 82 	200 82	201 82	202 82	203 82	204 82
        205 82	206 82	207 82	208 82	209 82	210 82
        211 82	212 81	213 81	214 81	215 81	216 81 
        217 81	218 81	219 81*	220 79	221 06	222 06
        223 99	224 99  225 99  226 99  227 99	228 99
        229 99	230 99	231 99	232 55	233 55	234 53
        235 53	236 53	237 99	238 99	239 99	240 99
        241 99	242 99	243 99  244 99  245 99	246 99
        247 99	248 99	249 99	250 99	251 99	252 99
        253 99	254 99	255 99	256 99	257 99	258 99
        259 99	260 99	261 99	262 99	263 99	264 99
        265 99	266 99	267 99	268 13	269 13	270 13
        271 13 	272 13 	273 13	274 13	275 13	276 13
        277 13	278 13	279 13	280 13	281 13	282 13
        283 13	284 13	285 13	286 13  287 13	288 13
        289 13	290 13	291 13	292 13	293 13	294 13
        295 13	296 13	297 13	298 13	299 13*	300 13*
        301 11	302 11	303 33	304 33	305 33	306 33
        307 33	308 33	309 33	310 33	311 33	312 31
        313 31	314 31  315 31	316 31	317 31	318 08
        319 08	320 08	321 08	322 08	323 08*	324 08*
        325 06	326 06	327 06	328 06	329 06	330 06 
        331 06 	332 06	333 06	334 06  335 06  336 06
        337 06	338 06	339 06	340 06	341 06	342 06
        343 06	344 06	345 06	346 06	347 06	348 06
        349 06 	350 06 	351 06	352 06	353 06	354 06
        355 06	356 06	357 06	358 06	359 06	360 06
        361 06	362 35	363 35	364 35	365 35 	366 35
        367 35	368 35	369 35	370 35	371 35	372 35
        373 35	374 35	375 35*	376 35*	377 33 	378 33 
        379 33	380 33	381 33	382 33	383 33	384 33
        385 33	386 33  387 29	388 29	389 29	390 29
        391 29	392 29	393 29  394 29	395 29	396 29
        397 29	398 29	399 29	400 69	401 69	402 69  
        403 69	404 67	405 67	406 67	407 67	408 99
        409 99	410 99	411 99  412 99	413 99	414 99
        415 99	416 63 	417 63	418 63	419 63	420 63
        421 63	422 61	423 61	424 61	425 99	426 99
        427 99	428 99	429 99	430 99	431 99	432 99
        433 99	434 99	435 99	436 99	437 99	438 99
        439 99  440 25	441 25	442 25*	443 23	444 23
        445 23	446 23	447 23	448 23	449 99	450 99
        451 99	452 99	453 99	454 99	455 99	456 99
        457 99	458 99	459 99	460 99	461 99	462 99
        463 99	464 99	465 99	466 99	467 99	468 51
        469 51	470 51 	471 51	472 51	473 51	474 51
        475 51	476 51	477 51*	478 39	479 39	480 39
        481 37  482 37	483 37	484 37	485 37	486 27
        487 27	488 27*	489 25	490 25	491 25	492 25
        493 25	494 25	495 25	496 25 	497 25	498 25
        499 25  500 25	501 33	502 33	503 41  504 41 
        505 53	506 53	507 53	508 53*	509 29	510 29
        511 29	512 29*	513 27  514 27	515 27	516 45
        517 45	518 81*	519 79  520 55  521 99	522 99
        523 99	524 99	525 99	526 99	527 99	528 99
        529 99	530 99	531 65*	532 63 	533 63	534 63
        535 63  536 63	537 63	538 63  539 63	540 75 
        541 75  542 75	543 75	544 75	545 99	546 99
        547 99	548 99	549 99	550 99	551 99	552 99
        553 99	554 99	555 99	556 99	557 99	558 99
        559 99	560 99	561 99	562 99	563 99	564 99
        565 99	566 99	567 99	568 99	569 99	570 99
        571 99	572 99	573 99	574 53*	575 99	576 99
        577 47	578 47* 579 45	580 39	581 99	582 99
        583 99	584 99	585 99	586 63*	587 99  588 03
        589 99  590 99	591 99	592 99	593 99  594 99  
        595 99  596 86	597 86	598 86*	599 84	600 99	
        601 99  602 71*	603 71*	604 71*	605 71*	606 71*	
        607 71* 608 69	609 69	610 69	611 69	612 69	
        613 69  614 69	615 69	616 69	617 69	618 69	
        619 69  620 69	621 69	622 69	623 69	624 69 	
        625 69  626 69*	627 15	628 15	629 15	630 15	
        631 15  632 15	633 15	634 15	635 15  636 15 	
        637 15  638 15*	639 15*	640 15* 641 15*	642 13	
        643 13  644 13	645 13	646 04* 647 02	648 48	
        649 46  650 50  651 50*	652 48  653 48	654 30*	
        655 28  656 28	657 28	658 28	659 18* 660 16  
        661 16  662 16  663 16  664 16	665 16	667 38  
        668 38  669 38* 670 38*	671 38*	672 36	673 36	
        674 36  675 36  676 16	677 16*	678 14  679 14
        680 98  681 16  682 16  683 16  684 16*	685 16* 	
        686 14  687 14	688 14	689 14	690 14	691 10*  
        692 10* 693 09  694 09  695 09  696 09  697 09 
        698 09  699 09  700 18	701 18  702 18  703 18  
        704 18  705 18  706 18	707 18  708 18  709 18  
        710 18  711 18	712 18	713 18  714 18  715 18  
        716 18  717 18	718 18	719 18  720 18  721 18  
        722 18  723 18	724 28	725 18  726 18  727 10	
        728 14  729 14	730 14	731 12  732 12	733 12	
        750 10  751 10	752 03  753 03  754 01	755 01
        756 07  757 07	758 07  759 07  760 05  761 05	
        762 05  763 05	764 92* 765 90	766 72  767 72
        768 72  769 72*	770 72* 771 72* 772 70	
    

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