I think the anti-theft idea is likely. Looks like a Cal. license plate number of that time, still six digits, 3 numbers then 3 letters. Stamping the face tells the guy looking for wheels to steal that they are going to be hard to sell. Using the plate number would make finding the rightful owner to be much easier.
All you need is some replica 1972-1973 CA plates for car shows...then there is a 'reason' for the stampings on your wheels.