Yes, 15 is 15, 503 is 66, 221,222,301 take the right one and that's all.
201 takes a 66. Some of the 600s take 66 and Touch and Sew takes the screw-apart clear, plastic ones. There was a lovely chart not long ago on QB. Try searching in the archive. It had pictures. I printed it out, put it in a notebook along with downloaded sewing machine manuals for future reference. I save stuff.