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)

cause: HAMMER used to free up stuck freewheel ratchet.

and yes, just about any freewheel will work... staying with the same tooth counts should make the swap simple.

i advise taking the wheel to a shop and having them remove the old freewheel.....

The damages could been cause by anything , the worn chain , a stick got caught in the chain ,or whatever . If you can fine a replace cog go ahead and replace , but since the chain was so worn it be best just replace the freewheel as a unit . The name brand would be found on the cone . (the inter part of the freewheel .) which doesn't move when the freewheel is spinning .

Most any 5 speed freewheel should work.
I don't know if there are Italian 5 speed FW's, but stick to a "Japanese" compatible one. Shimano, SunRace, SunTour, SRAM....

Don't sweat why, (35 tears old parts?Forced shifts climbing hills?) just buy new parts.. new freewheel and chain go together...

I've broken teeth clean off. I haven't priced a freewheel in a long time. I switched to freehub some 30 years ago by
buying a hub and cassette and building the wheel with the same rim and spokes. Unless you like riding a museum
piece it'll work better in the long run.