In the '70s there were frequent public service announcements on from the various PDs who would loan you an engraver and encourage you to put your SSN or your Driver License number in a conspicuous location on your valuables.
Seems he was following that advice.
Per the previous post, in boot camp (Navy) we stencilled our SSN directly on our sea bags, skivvies, uniforms..
I just sold a trunk this past year where I had grind that off of there.
Seemed like a good idea at the time...who would've thunk someone would steal your ID?