As MY pointed out--the bottom end of the spring ends in a pigtail -- and the specially shaped washer with built in stud locks it in place directly onto a steel pad on the trailing arm (most likely the washer you listed). This part does not need to be replaced unless the stud is broken. Sometime the stud will spin in the washer but that can be fixed with a spot weld. The spring "seat" should be the rubber cushion that fits over the inverted cone which locates the spring at the top--good part to renew. Sometimes the inverted cone gets rusted out (held in place by a centrally located single bolt--which amazingly usually comes loose with no trouble). Good cones can be sourced at a wrecking yard and would be a good solution if yours is (are) bad. -- Dave