pinkCastleDH,
Only the 66-1's used the back clamp feet. After a few years singer switched to the side clamp or low shank design and from then on the 66- ...... use those. There were far more later series -4 red eyes made than the -1 models. I've seen only a few -1s but many many -4s. I guess it might be a geographic thing.
makitmamma,
All you have to do to run an electric machine on a treadle is put it in the cabinet, remove the motor belt and put on a treadle belt.
That's it. Then you have a dual use machine.
Now, if you want the rotational inertial of the larger spoked hand wheel, you'll need to swap the small solid hand wheel for a spoked one. They do make a big difference in treadling. Also the belt cover / bobbin winder might need changed too. All those things can be had.
Joe