I wouldn’t think so. It looks a whole lot like an early (ac42 early c block or earlier) Walther slide cover. If it were Mauser, it would have been stamped with a e/135 or e/WaA135. If it were an ac44, I would have thought that an early part simply got mixed in the slide cover’s bin, but it’s a Mauser pistol. My best guess is that the original unmarked slide cover was damaged or lost. The one on it now was either obtained during the war or post war and replaced the original unmarked Mauser one. It would not have come from Mauser like that.
Looks like a really nice P.38. I have a little earlier y block byf44 than yours. Out of all the P.38’s, the byf44’s have survived in the nicest in condition.