6-cyl harmonic balancer/pulley/rubber broken

[Nevada51]

Not sure what the part is, so was hoping someone could point me in the right direction as to what I need to order. See pic.

Background: 1964 c10 with a six cyl, stock engine, fwiw. I noticed a rubber/seal part on the crank pulley seems to be failing, and has a crack in it. Engine sounds fine at the moment, but what part is this? Seems to be between the pulley and harmonic balancer? I’ve circled it in blue in the pic so you can see what I’m referring to.

For bonus points, would this be the same part for a small block V8? (just thinking ahead in terms narrowing down what I need to order.)

I need to order a workshop manual regardless, but I’m not 100% sure this would be covered in it anyway. I know I need one regardless…

Any help appreciated!

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