I think what probably happened was GI must've brought back at least a couple (GI's are crafty - sometimes bring back a couple of the same thing in case one gets taken). Somehow the paperwork for the other one got stuck with the pistol you have. You could inquire to the previous owner about the other Luger that matches the paperwork - he might still have that one or know where it is.
Imagine how grateful the owner of the other Luger would be to get the bringback papers for it.
