Hi, welcome to the forum.
When collectors talk about matching parts, it means that all of them are numbered by the factory as the pistol is manufactured and fitted.
Your friend's Luger appears to have been assembled from a mix of found parts. It didn't leave a factory that way.
There was some activity by civilians with access to parts to fulfill the demand created by GIs that wanted to bring home Lugers. They may have come from a repair depot or other level armory.
The receiver was made in 1941 at Mauser's Oberndorf factory. The toggle's "42" is a manufacturer's concealment code for Mauser from earlier in the war.
This is a family heirloom that your friend should be proud of. A real memento from the father's service in Europe after the war.
Marc
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 Igitur si vis pacem, para bellum -
- Therefore if you want peace, prepare for war.
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