I can't answer the question precisely, but it's not a cleaning rod nor is it a stacking rod. I found a copy of the German army manual for it (all of nine pages long) but I couldn't real the name. It almost looks like a gas regulator adjustment, though that seems doubtful.
Shoot low, sheriff. They're riding Shetlands!
Underneath the starry flag, civilize 'em with a Krag,
and return us to our own beloved homes!
Buy War Bonds.