crankshaft pulley

The vibration damper is made up of two pieces of metal bonded together with a rubbery material. Years ago the drive pulley was bolted to the center hub of that assembly, but on yours, the pulley is cast into the outer ring. Because of the added stress from tugging on the belt, it is fairly common now for that bond to break, then the ring / pulley slides off-center or falls off.

You can get a used replacement at any salvage yard, or a new one at any auto parts store. The auto parts stores have books showing this part for a whole pile of applications. Because there are too many to stock, they will likely have to order it for you. Typically it will arrive in one or two days.