Marc, I don't have data on frames, but there are other pieces that are Armorer Replacements that can be documented. I have seen Lugers with badly beaten up trigger guards, that frames easily could have been replaced by an Armorer. The S/42 marking, along with the acceptance marks would certainly lead one to believe it was a factory replacement.
The bothersome part is the sloppy work with the serial number. That is not typical of the craftsmanship of experienced armorers!
While you may have a historical pistol, it is going to just have to be an interesting piece. Tough to claim it as a collectable G-Date <img border="0" title="" alt="[Frown]" src="frown.gif" />