On the second page of the lommen link (first link) above shows all the mechanical doodads that link the RF to the rails.
My Anniversary Speed has the rail thingamajig riveted to the rails. I think some cameras have it attached via screws in threaded holes at a precise position on the rails.
Being the gismo is missing on yours and you don't know what to look for buy another complete camera and use this one for parts or look for a set of rails with the gismo attached.
Finding the rail gismo separate somewhere is an extremely long shot.