It does look like someone used tools around the hole
the shiny spots above and below, where a rod, drill or screwdriver rubbed against the metal. There is also some discoloring inside like JB weld or something else was stuffed inside to close the hole. And the slide thing?? Maybe the guy was using incorrect powder and needed to vent more gas and speed up the bolt. In the 50s these things were $20 items and if you had $100 worth of stolen government powder?????
Inland: Thanks for the lesson, I should have gotten one out and looked at mine.