Missing part on my Alenco window

I have an Alenco window with a block and tackle system. The window has missing a black plastic piece on each side of the window. When closed the channel balances rise up thru the opening . They appear to keep the channel balances in place. Are they called Balance tops? or Face guides? I have a good window with the items in place, but i am reluctant to pry them off to see their construction. Channel depth is 5/8 and width is 9/16 and a 28 in length. Thanks for your help. Floyd