The last available part no. for these models you describe was 159056G for 3 belts and 159056A for 25 belts, but this went out of print recently.
You'll have to buy current part no. 301291 (301291G for 3 belts, or 301291A for 25 belts), however, you will also need to stretch it out with heat (douse it in warm water) to make it work right with the older 500s.