I have a 1988 Miyata 312 with all original components. The rear derailleur, a Shimano RD-A350 with 6-speed SIS index shifting is broken, and needs replacement. Sheldon Brown says that almost any post 1997 Shimano RD should index properly.

https://www.sheldonbrown.com/dura-ace.html#indexing

It would be comforting to get some verification of this.

While I would prefer to retain period correct parts I would not be too averse to using a modern Shimano derailleur such as a Tiagra.

What are some suggested options?

The only ones to avoid are the Dura Ace derailleurs pre-9 speed or 1997.

Otherwise, any derailleur from an index era Shimano group should work.

The actuation ratio hasn't changed.

I had that same derailleur break last week, and replaced it with a RX100 out of my parts bin.

Was it the little plastic thingy that holds the coil spring that provides side tension in place that broke?

I had just finished replacing the chain and was testing the shifting when all of a sudden it worked no more.

I've got one of those.

Nearly all new low-end derailleurs are SiS compatible. If you don't have ready access to used parts, you can pick up a Tourney or a knock-off (Sunrace RD-M2T, Falcon Power, etc.) that will work. A seven speed derailleur will work fine so long as you adjust the outer limit screw to keep it from overshifting.

You read Sheldon Brown and you're still not convinced that derailers don't have speeds?

I assume you are referring to me since I'm the only one to mention Sheldon Brown.

I assure you I understand that "derailleurs don't have speeds". I don't think I ever implied that they do. I certainly didn't mean to. But shifters combined with derailleurs "do have speeds" and I just wanted to be a little more comfortable if I chose to go with a modern derailleur.

BTW, I swapped out a Shimano Tiagra RD4500 SS from my Soma Stanyan onto the Miyata and it works great. I have decided to go that route.

Thanks to all for the input.