Wow, I should go check the three I have in my garage!
Some of those cams will most likely work in your 503. Note that the feet will not work on a 301a, as it is a slant shank machine.
I will I'll look at my machines tomorrow and let you know if I need anything for them. Possibly a new home? No, just thinking out loud there...the 600 and 601 have metal gears and are worth keeping, but I don't know about the later models.