The "DP" on the extractor is a red flag to me.... That's a Brit marking indicating the part is only serviceable for Drill Purposes. Sometimes DP marked parts were merely obsolete, sometimes they had been damaged (i.e. by fire) to such an extent to make the arm unserviceable. I would have the reciever checked for proper hardness even if a DP marking couldn't be found anywhere on it. May well have been milled off.