Washer for control arm?

A quick learner from Virginia says:
While installing a water pump just outside the window, my hand slipped and popped the control arm out of the window slot, losing whatever washer was atop the pin. I have a separate lock washer that fits the pin, but I can't get it to keep from dropping out of the window's slide track. Any ideas of how to secure it?
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Emily from SWISCO responded:

Hi there!

Your operator resembles our 39-253LB or 39-253RB, which uses the 39-529 track slider. Compare our dimensions to yours and see if this matches what you need. 

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