This spring fits around the pop bumper plunger and pushes the ring and rod assembly up.
Also used on many Williams flipper mechs as well as other various assemblies.
Williams/Bally reference #'s 10-135 & #10-300.
Stern/Sega/Data East reference #266-5047-00, 266-5020-00.
Jersey Jack Pinball reference #13-007002-00.