This ball bearing is placed in the socket hole on the side of your Fender System 1 arm module outer sleeve and held in place by the spring steel retainer ring. It allows the arm to snap into place.
It can be a real trial getting these on, there is no tool or trick. I get the outer sleeve on my thumbs in both hands and pull the ring down over the BB using a fingernail on each side. Don't be afraid to give it a good stretch, it's spring steel and will snap back into shape.
Fits only the Fender Contemporary System 1 tremolo found on Made in Japan (MIJ) Stratocasters, Telecasters and Squires that were made between 1985 and 1987.