"O ring, Caterpiller
I guess I found my out my own question, Sorry to waste your time. I
found two different numbers.

It is a fat yellow O ring, Caterpiller part no. CT 8M 4991. They are
about $3.00. It measures about 1 9/16" OD, 1 1/8 ID, 7/32" profile.

Caterpillar Part #8M5266. Installation is simple, place the O-ring
on
the right bar-end. When ready to engage the throttle lock roll the O-
ring towards the throttle grip until it rests in the valley between
the end of the grip and the bar-end. The O-ring does NOT fall into
the valley, it simply rides on top of the valley. It also creates
enough friction to hold the throttle in place."

Caterpiller O-ring Specs:
Inner Diameter = 0.984 inches
Cross Section Diameter of O-ring = 0.210 inches.