It would be interesting to see how the grinding/polishing marks run out on the obstructions on the receiver. On the bolts the grinding/ polishing marks look like hey were put on with a cylindrical grinder then the areas around the obstructions (locking lug, bolt handle and guide) look like they were done on a surface grinder with a radius cut into the wheel to match the OD.
If the marks on the receiver are the same, it could have been some experimental armory project. If Bubba just ground and polished off the original stamping, he got through the case and the underlying metal is butter soft and easy to stamp. Maybe Bubba was afraid of holding "stolen government property" and wanted to the remove the evidence?

Just a funny anecdote, I had a customer that had restored a 30s model 1 ton Ford pick up and needed to sell it. To sell it so it could get a license plate to be street legal it needed a serial number even though as sold by Ford it had none. I lent them my set of stamps so they could put  a random number on it.