A quick learner from Ellicott City, Maryland says:
February 24, 2017
This part screws into the top of the window and stops the
top of the channel balance. And holds it in the track. It has a flat top which I
failed to take a picture of.
Tom from SWISCO responded:
March 1, 2017
Hi! Thank you for reaching to us. Please take a look at our 18-009 top sash guide. Make sure you compare your measurements and dimensions to ours and let us know how this works out for you.