Bike is a '78 Nishiki International, all original components so far as I know. The chain was starting to skip under load, so I checked for stretch, and yup, super worn out. Put on a new chain and tried it out, and the skipping was WAY worse- happening constantly, no matter how little force I used. Tried shifting gears, and the issue disappeared. So I took a look at the rear cluster, and there were five broken teeth on one of the cogs! Now I'm just amazed it worked as well as it did for as long as it did...

Anyways, two questions:

1. What the heck could have caused this?

2. Obviously I want to replace the cluster- can I get any old 14-28 5 speed cluster and swap them out, or are there compatibility issues I should be aware of? (I couldn't find a brand on the current cluster, though that may just be due to dirt)