71 Duster wiper motor question

[Avispa]

I took the wiper motor out today, three nuts in the engine bay and a 1/2 nut on the wiper motor, there was a black plastic washer looking thing on the backside that fell off when i took the arm off, anyone know how it goes back on? looks like something to do with the parking, but not sure, thought that was internal to the motor.