Uncommon enough that I did not know about it until Myk Daigle contacted me with an odd pen that might suit my taste. This model didn’t even make the article on Hump fillers for Paul’s Fountain Pen Journal.
The Betzler hump filler pen utilizes a notch in the base of the hump to catch the end of the barrel slot. The lock is a simple end knob. The pressure bar is perfectly sized so that when the knob is screwed in, the hump is moved forward slightly and locked to the end of the slot in the barrel. A little loosening of the knob and the hump can move enough that it can be depressed.
This shares the elegance of the Wirt hump filler by using a hard rubber hump. The lack of externally visible metal locking parts is a nice touch, as is the surface treatment of the knob. And the hump is very small and unobtrusive. This is an attractive feature but would do little to help brand the pen in an interesting way.