Jim Thompson wrote:

Does this exist...

http://analog-innovations.com/SED/BallAndSlot.pdf

Ball and _Slot_ as opposed to the usual Ball and Socket.

Low load. Hobby size... maybe 1/4" to 1/2" ball.

...Jim Thompson

Looks like a standard "T" bolt slot to me ! Like on lathe or machine
beds but with a radiused bottom.

--
Best Regards:
Baron.