Will the Walther parts fit the Mauser P38? Also, will the new Firing Pin Retainer Pin work for the old style pin in the War era pistol?
Yes, the Walther P38 parts will fit the Mauser produced P38. During the WWII, the Mauser factory began producing P38 pistols because of the increased demand by the German army. The manufacturers stamped the P38 pistols with secret codes. The Walther produced P38 had a secret code of ac. The Mauser P38 was stamped with byf, which in 1945 was replaced by svw. There was a third manufacturer of P38 pistols, the Spreewerk factory, which used the secret code cyq.
Please note that the new and old (WWII) parts are not interchangeable, and the new style parts will not work with WWII Era P38 pistols.