trying to identify disc sander part and find replacement

I have a old dept 12 in disc sander. Not the original motor. I need to know what this part is called and where to find one. It is what the disc connects to and the u attach this part to the shaft of the motor.
It is 4 inches across the widest part and the hole in the middle is 3/4 inch.

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trying to identify disc sander part and find replacement