This is a rarely seen armorer's guage for measuring the opening of a K98k metal muzzle cover.  Part of the function of the metal muzzle covers was to protect the muzzle when the rifle was being cleaned with a cleaning rod.  When the narrow end of this tool could be inserted in the opening of the muzzle cover, the cover was worn beyond specification. I believe RLM means this tool was made for the Luftwaffe. The waffenamt was applied with a rotary cutting tool and was probably pantographed.  Even the swastika is visible.