THIS looks like the gasket and overpressure plug for yours.

Fits Presto Pressure Canner models: 01/CA16A, 01/CA16H, 01/CA21A, 01/CA21H, 21, 21AV, 21B, 21S, 7, 7AV, 7B, 7S, CA16, CA16H, CA21, CA21H

However, I've seen photos where a 21b doesn't have a plug, it has some sort of other contraption that I'm sure involves a spring. I don't know which one you have.

There's a nice write-up on refurbing a 21b here: http://cookingwithjim.com/presto-21-b-canner-refurbishment/