I was able to acquire new old stock Gemtech endcaps from Dr Phil Dater of Gemtech before he passed away.

I bored some out and threaded them to leverage modern ‘hub’ thread (1.375×24) mounts.

Pictured below are two of my favorite use cases.  I like the Bixler 3 mount the best as there are no springs to fail and it mounts up rock solid unlike spring loaded 3 lug mounts.  The Griffin A1/A2 FH mount works just like the old Gemtech HALO mount which I think was invented and licensed from Parker Hale from England which is no longer in business.

 

While less common than Hub threaded devices, I made a few for those that have Alpha thread pattern devices.