I recall hearing of a pump gun that, when trigger group was re-assembled, if the bolt and/or hammer wasn't in the correct orientation it raised havoc. The R-31 may have its own foibles. My old copy of
Firearms Assembly: The NRA Guide to Rifles and Shotguns comments about the R-31:
When reassembling the mainspring, mainspring follower, safety spring, and safety plunger, it will facilitate reassembly to insert these parts first. Then, after replacing the mainspring pin, and using a short length of wood dowel to compress the safety plunger, re-insert the safety. This method is converse to logical procedure—that is, first replacing the safety, then spring-loaded parts.
Someone could have had trouble reinstalling the safety, given up, and sold the gun. Unfortunately, the owner's manual doesn't have dis/re-assembly instructions; but, if you don't have a manual, for a .pdf, click
here and select the Model 31.