I would like to know a source for the nut with the 3 screw holes.
Barry Young, who is an APUG member, made a couple of them for me. His prices were very reasonable, and he can machine them to order. I used them to add second tripod sockets to my 11x14" American Optical and 7x17" Korona, so that I could use them with Arca-Swiss style plates without worrying about the camera twisting on the plate and for extra rigidity. Now I can turn the Korona sideways for verticals.