The service number became the SSAN sometime in the late 1960's, I think. Prior to that, you were assigned another number when you enlisted/were commissioned.
Military vehicle "bumper numbers" (that's what they're called) are the military official utility vehicle replacements for license plates numbers - which is why they're found on both ends (typically on the bumpers themselves, on a flat surface.) The numbers are stencilled on, not decals.
You can probably Google <bumper numbers>, or check to see what you can find at steelsoldiers.com or armyjeepparts.com for guidelines.
That being said, you can still do whatever you want. I believe bumper numbers are six or seven digits - but don't quote me on that, see what you can find out!
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