Compare the corrosion to the No4(T) found in the wall of a house in Arnhem. And to the No32 found in a nearby ditch decades later. Not the same at all, except that the corrosion on the rifle and adjusting tool are commensurate with being poorly stored in a building for who knows how many years.

Several possible scenarios: canny local finds dead sniper when the front moves on and relieves him of weapon, case and tool. The sniper wouldn't bother to carry the damn case around, just the tool, perhaps.

Or, front line moves back and sniper is forced to ditch his weapon as a prelude to escape or capture and stuffs it somewhere he thinks it won't be found, and it isn't, for some years.