Hi Glen,
The thanks should go to Martin Krause, firearms author, who lives in Germany.
He emailed some collectors a composite listing of all patents from all countries associated with the Luger.
I just found a web site which lists many of the US patents for the Luger without having to pay a fee for access to the patent documents.
Unfortunately, these pdf-files on the web site are protected and one can only view...not print.
Glad your friend has an answer. I am sure his stock might be quite valuable
. I would have him look into the BATF legalities of attaching one to any Luger...
BTW...any chance your friend would allow you to take photos for posting. I and others would probably love to see one "in person"...or at least in photos !!!