My guess is that it is a serial number. If it is not the same number as your rifle,
the assembly is not originally from your weapon.........
It could also be a privately engraved ID number from a previous owner for insurance purposes. I've seen this before on many different things in the past. This one was likely engraved Inside the well, so as to not be disfiguring to the piece itself. Although I've never liked the idea of permanently engraving ID numbers onto historic items, people did used to do such things a number of years ago-half the time never figuring that the item they were marking thus would ever be worth being a collectable piece. Who, years ago, would ever have thought that a run of the mill M-1 Garand would Ever be a collectors item? William
"Much that once was, is lost. For none now live who remember it."
Is it the veteran's service no.?-the engraving doesn't look like it was factory done.