they also may be stamped P-484-[47, 48, 49] ... possibly without the P and/or dashes.
you have a machine that is missing the discs?
it may be hard to find the exact ones you want and you'll have to get any 20 stop discs you can find and rewire the reel wiper boards to match your reel tapes.
doing that will change the payback percentage of the game, but that may be a good thing :-)
if you know someone that can cut metal with a cnc/water jet/laser, the specifications to make a disc are available and probably understandable to machinists. If you have at least one disc, that'd help as a sample to confirm the dimensions.