Marc, I have no idea. It’s the third A5 I’ve had that has had it. It will sand off pretty easy. I wonder if there was a slip on recoil pad installed at one point. Maybe moisture, sweat, oil caused a reaction with the rubber and finish?
I just picked up a 1928 with this exact same marking on the butt stock. The butt plate looks really (!!) good, as does the rest of the gun, but the rear 1 -1 1/2" has discolored much like the first pictures in this thread. I am deciding if I want to work on it, or leave it alone. I'll probably leave history alone.
