As for the boxes, I'm sure I've seen attachment box numbers on some of the parts charts. Where I saw it though, I don't know if I could locate them again. Maybe start here:
http://www.brewersewing.com/BQSS_Consumer/b2bsewing.asp
As for what goes with what - for the most part low shank attachments will go with almost any of the low shank machines. Straight stitch and zig zag machines will prefer different feet in some cases, but a low shank straight stitch Singer will use almost any low shank foot for a straight stitch machine whether it's a VS, 66, 201, 15 or what have you.
Slant feet will only work on slant machines, but they don't even fit the Low shank machines. Low shank feet could be installed on a slant machine but will not line up properly with anything and may be hit by the needle and cause all sorts of calamity.
So in short (I know, too late
) as long as you know if you're getting low or slant shank feet and which of those 2 you need, you're good.