Sniper!

This is a 500mm lens by Leitz, I understand, and the fitment is a Leica screw, which looks tiny at the right-hand end of the lens.

A fellow member of the Bridport Camera Club is a keen adapter of various lenses to different bodies, and this is in his possession for the moment.  Something similar from Russia, ages ago, was called a Sniper.  Or something like that.