Winchester received 5,000 recoil plates from Underwood
in Nov 43 and my 5.6 has one of them as original equipment. It is marked AU on the right side of the tang. You may have one of these. Mine was built in May of 1944 and they apparently dipped into those 5K at this time. How does the stock grain look under the plate? does it reflect the metal etchings in the wood? Mine clearly does and the plate was a real bear to remove from the stock which clearly shows the pattern of the metal in the wood. Good luck.