You are right the strap / sling mounting is the same thing, cross cut under the barrel probably done on the same machine that cut the grouve for the front sight..
the stamp is not a antler but an eagle with spread wings.!! with the head facing left and the wing tips up as high as the head of the eagle. and it looks like IV but they are stamped almost on top of each other ( the letters)
The same sign is on the barrel and the breech next to each other.. im sure it's IV the top and bottom of the I are over hanging to the left too far for it to be an E and the top of the V is in the middle of the I
I would attach a pic but I do not know how to... does this help?
ahh just spotted it again on the bolt.. eagle and IV serial number is 2286## last two are numbers obscured for my privacy.
Colin