I have one of those positioning plates. I can't recall if it's for the 301 or the 221 though. If they're mated with the right throat plate, I think they can be interchanged between the FW and 301. It's largely the position of the holes drilled in the throat plate that make it all not work right.
Here's why I think that:
To the best of my knowledge, the 221 didn't come from the factory with a numbered throat plate. The 301s did though. I wanted a numbered plate for my 221, and dropped into the Singer shop that used to be here. She sold me an original numbered plate but when I tried it on my machine at home with my original positioning plate, it didn't work - the positioning plate didn't work right (it was too far in one or the other direction, and didn't catch the finger) - I went back and we swapped the positioning plate with the original one from the numbered plate and it worked on my FW. Still does to this day.
Also try Glenn Williams in Florida. His prices are great and I've dealt with him a few times for parts and he's really friendly and knowledgeable.
Some other people online charge a fortune for parts that aren't really that rare.