[John Sabato]

They CAN be Interchangeable but often require fitting by an armorer. Once all the parts on a newly manufactured Luger were fitted, and inspected they a were numbered alike to make sure all the correct parts were assembled after the bluing process. So if a numbered part had to be replaced the substituted replacement part would have to be fitted to work properly with the rest of the gun. Sometimes you get lucky and a replacement part would drop right into place and be functional. Other times you might have to try several parts that were a close fit and then polishing would complete the job of restoring functionality. It is why the Luger was so much more expensive to manufacture and maintain.