In Germanyicon today the slide is the numbered/controlled part, not the frame. A lot of the post war surpluss P.38 & P1 pistols that have come into the US have had to have a frame nr stamped on them to meet US law.
Thus a GI 45 with the frame nr removed would be legal in Germany. The owner would just have to stamp a number of his choosing on the slide.
Sarge