Guess I can state the obvious for everyone...

That receiver is only 5124 into QHMC's first serial block. If I am reading the picture correctly, the Rockola barrel is stamped Feb 43???

The story of the stock is interesting, but your friend has a wonderful piece of history. I am with Dbarn, I am curious as to other marking and parts to the carbine. Your friend may have something special to go along with that homemade stock.