The part I mean is the metal plate that is built into the cabinet, and springs up when you lift the machine up into working position. It looks like this:
[ATTACH=CONFIG]548456[/ATTACH]
It is located right under the balance wheel, and the belt goes up through one hole and down through the other. Here's someone else's machine where you can see it:
[ATTACH=CONFIG]548457[/ATTACH]
I actually have the metal plate, but underneath it is a strong spring with a sort of long "finger" that stretches out underneath the plate. When the machine is folded down, the weight of it holds the plate down, but when it's raised the spring pushes it up so it lies flush with the table top. On my cabinet, the end of the spring finger was broken off, so the plate just hangs down. All i really need is the spring, but try to find them!
I actually have a 201K (first vintage machine I ever bought!) and it also has this spring/plate mechanism. Only since it's electric, there's only ONE hole in the plate, for an electric cord to pass through. The springs are all the same.