The XXI is the Fascist year from the start of the regime.
RE, RA, etc would be the Italian branch they were issued to. Not sure on PSE.
Those are very common guns, unless it had highly specific info on it i can't see it going more than $400. If it was accepted for service by the Third Reich (and they used just about everything they could find) there will be waffenamts stamped on the gun in multiple places. Looks like an Eagle sitting on a swastika. That's the acceptance mark for German service.
Those can also be faked, so be wary.
"And Shepherds we shall be. For thee, my Lord for thee. Power hath descended forth from thy hand, that our feet may swiftly carry out thy command. So we shall flow a river forth to thee and teeming with souls shall it ever be. In nomine Patris, et fili, et Spiritus Sancti. Amen."