My old one (around 48-49)
and my "new" one. A 10-5 from about 64-65
BOTH of them have cryptic numbers written on them with an "electric pencil". I would presume they are police department inventory numbers.
I have wanted one for years. They are just the neatest things. Nothing like a 2" M&P. My grandfather carried a 2" M&P 10-5 when I was a kid as a deputy sheriff in the 70's with grips he made himself with a thumb shelf. I have seen the bill of sale where he got it at the hardware store. If I remember correctly, it was like $110-$150. Something like that.