Swerve, we had FR followed by a seven digit number for regulars and FV with a seven digit number for reservists. As you point out, the Social Security Number (which was not to be used for identification) was then adopted for use as identification, an event that to this day causes problems for retirees.
The DD Form 214 which we were all advised to register at the local courthouse upon retirement is, as a public record, now being retrieved by identity thieves who use its information which conveniently includes the retiree's SSN.