The old 66s used a back clamp foot where the screw entered the shaft from the rear.
The White machine that I am talking about uses what I've read described as the top clamp. The foot has a two pronged fork that slides onto a shaft under a big knurled nut that is screwed down tight to hold it.
Here is a pic of mine with a close up of the top clamp foot. The foot that's on it is an unidentified hinged foot we got from our LSMG. It works but there is little clearance under it.
Joe