It looks like it might be a "Magic Muffler"?
A few yrs. ago I bought a box of K&B 6.5 stuff from a guy on epay...there was one of those things in there and that's what he called it.
The stinger on mine is threaded and adjustable for length.

Edit: Mine doesn't have a header though...it just slips over the O-ring on the rear exh. header adapter typical of the K&B 6.5