Missing part on my Alenco window

A quick learner from Boulder city, Nv says:
August 24, 2011
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
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Casey from SWISCO responded:
August 25, 2011
Sounds like a top sash guide. Take a look at 18-174, if it does not match up to the originals look through the top sash category section or post images with dimensions for us to research.
5/8" Top Sash Guide, Alenco
5/8" Top Sash Guide, Alenco
18-174
$1.96
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