How many types of Singer/Graflex rh-50 holders were produced?

The ones I have or have good photos of seem to all be either (1) 4x5 Grafloks, which can be converted to the XL type; or (2) XL holders which can be converted to 4x5 Grafloks. I have found none for the 2x3 Graflok, or any other Graflex style.

To convert an XL type to 4x5 Graflok, just loosen the four captured screws at the corners and remove the adapter. The felt seal has no effect, film plane distance then matches exactly my other holders.

To convert a 4x5 Graflok to XL, find someone who has removed the adapter plate, attach the adapter plate, and, if absent, stick some felt pad material to the area between the inner opening and the inner edge of the adapter plate.

Correct?

I think you're right about there being one "RH-50" with two mount styles. Pacific Rim Camera's posted information is that the device was supplied only for "XL" and 4x5 "Graflok" backs, as you speculate.

For four decades, I've thought there was one for 2¼x3¼ "Graflok" backs, too, but I have no evidence at all that there ever was!