Have a look to see when the barrel was changed my 1916 5MD was changed to a heavy barrel and was stamped on the butt wrist as such think that happened in 1937 for a lot of them for some reason here is a couple of pics it still has the original wood mine is a 4/16 build, it wears a Lithgow5MD 1916 '07 bayonet which with the 5MD Lithgow 1916 Scabbard plus a black (not blackened) Wright 1916 sling which together cost me 5 1/2 times what the rifle cost which was a a miserly $100 for the rifle. But it was missing the mag & safety & sling.
The circle with the question mark in the circle on the reinforce no one on this site has been able to decipher it I cannot find anything on it either.