Ok, so my 15-91 takes a class 15, as shown on the left. It has a flat top. The 503 takes a class 66 which has a slightly rounded top. The bobbin on the right only fits the 221, 222 and 301. Is that correct?
The middle bobbin is still not identified for sure. But I know I don't have a machine they will fit. If anyone wants them, you can have them - there are 4 of them.
Thanks for all the comments!