Since the firearms traceability regulation was adopted into national legislations a few years ago, every vital part of a gun crossing EU borders must have prior import and export licenses for every crossing. Exempt are firearsm coverd by the EU gun passport and accompanied by the owner.
Vital parts are the barrels, receivers and all parts needed to function as a firearm. Lock parts would most likely fall into this definition.
The permits apply to each border crossing, regardless of whether the firearm is licensed in the EU or not. So you will need a German import permit and a US export permit, then when the job is done a German export permit and a US import permit.
These regulations have effectively robbed EU firearm buyers of makers' warranties and no one noticed. It suits the makers of lemons for sure. Sane people would not undergo months of buraucracy and considerable expense to send a 1000 Euro gun to the makers for warranty work.
High end guns that should be serviced by the makers are not, for the same reason.
Bottom line, find someone in the US to do the job.
