This encoder is for Gammill longarm machines with regulation. This is the slip on encoder or non-locking. Gammill Plus model machines with the "curly phone cords" and blue connector as shown in these picures use this encoder. Each machine has two of these encoders.
This encoder controls the machine regulator.
Confirm your encoder has the 5 gold color pins as in the photos indicating it is this "Slip-on" encoder.
Encoder has been tested to confirm working order.