Hey oregongirl,
First you're correct, the green case is for a low shank machine. Your 503 takes one of the buttonholers in the pink or maroon cases.
The green case BH should work on any of the low shank machines that you have, they predate the zig zag machines. I have a green case BH that came with my 15-90. The only one I've had cause to use is the pink one on my 503.
The buttonholers work like the zig zag attachment for the same machine, they fling the fabric back and forth to make the zig zag part of the buttonhole edges. This video from April1930s shows it working on a featherweight:
http://www.youtube.com/watch?v=9XlkM_FOQos