“It is usually stolen guns which have serial numbers ground off.”
It is possible. It is just as possible that it is one of the millions of shotguns made before 1968 that were not required to have serial numbers.
The requirement for .22 rifles and shotguns to have serial numbers was part of the 1968 gun control act.
AND prior to the 1968 infringement, one could order a shotgun or small caliber rifle and have it sent to your front door via USPS. BUT, the sheriff’s dept did asseret that the serial number had been taken off.