I have a Heathkit IT-21 tube tester. I decided to replace the paper cap and noticed a "stack in a clamp" of small disks. Is this a rectifier, a voltage reference, a 2000 amp fuse in a 20 mil ckt, or what?
There's no space between the disks and I didn't want to disturb it. This is a very basic tester I know but it done me well for what I do.
What is this mystery part?

It sounds like a Selenium Rectifier.
Can you post a picture?

If you heat it with a soldering iron does it smell like rotten eggs?

I looked at the schematic and the thing is very simple, has very few components. The only thing that could be is the meter rectifier. It should have a red and yellow lead coming from it.

It's a copper oxide rectifier, judging from the look of it.

Thanks. I think I'll leave it alone for now. It's working for now but this thing is looking sketchy.

this is a good read about the selenium rectifiers. Also remember taking them apart to clean and putting them back together; in the right order.
http://w3hwj.com/index_files/RBSelenium2.pdf

This one isn't selenium; it's copper oxide.