And to add to the mystery. I just found what looks like a civie Ithaca with the same markings on a trench gun adaptor at a rock island auction. It says it was made for US army special forces. Theirs looks like it has stampings on the stock too. But my stocks are missing so I don't know if mine had any or not. Here is a link to the auction.

http://rockislandauction.com/viewitem/aid/50/lid/1550