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UNITED STATES v. O'BRIEN
SUPREME COURT OF THE UNITED STATES
391 U.S. 367
May 27, 1968, Decided *

[extract] The Selective Service number itself indicates his State of registration, his local board, his year of birth, and his chronological position in the local board's classification record.

Cite http://www.law.umkc.edu/faculty/projects/ftrials/conlaw/obrien.html

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Selective Service Number coding:

First number — state issued
Next two digits — local board
Next two digits — year of birth
Four group of digits — the registration number

From Page 248, The Complete Guide for CPP Examination Protection (pub 2006), via Google Books.

http://books.google.com/books?id=Qq2H3_1bWUgC&pg=PT249&lpg=PA248&ots=rwSRR4gpLy&dq=how+are+states+coded+in+%22selective+service+number%22&sig=ACfU3U1V9Mgk_Fqk84PHXsAkuU7IVbrIng#PPT273,M1