Notice in the photo of the 55-S RPM , that the underlying S bulges almost directly points perpendicular to the side of the torch, where as in the original coin, the bulge points at a downward angle towards where the side joins the knob of the torch, even though the "S" in both cases appears to be positioned the same.So I do not think it is a OMM ( D/S) nor a RPM ( S/S). Such would have a distinguishable shaped and sized structure, one over the other, which I do not see here. I would call it die deterioration.
What about this 1976 D penny looks like gold one but it's weight is 2.1 g even plated sould weight more than 3.5 g?