I think they may have just thrown darts at a line of numbers. "We'll call this one a ..." Plonk, plonk, plonk "...201! And this one will be...." plonk, *miss*, plonk "....a 99!"
They really don't make sense. I came across reference to a Singer 502 and thought it was either a typo and they meant 503, or maybe a Rocketeer I'd never seen. So I Googled it and the 502 is a Singer machine, but it looks nothing like the Rocketeers.
So:
500A = Rocketeer (and why the "A"?)
501 = ?? Google doesn't seem to be sure if these exist, I get hits but they're 301's, 401's, or 503's that come up
502 = Exists but definitely not a Rocketeer
503 = Rocketeer again!
Yep. I'm thinking darts.