I think this plastic piece is called a sash stop

A quick learner from Las Cruces New Mexico says:

One of my windows is missing this plastic stop on one side. When you push the window down the weight and balance stays up on one side because the plastic piece is missing. I need to order six of these in brown. Please send me a part number if you have them.

Measuring from the top side of the window it measures approximately 5/8 x 7/8.

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Tom from SWISCO responded:

Hello! This part is called a top sash guide. I'm confident that we carry a good replacement for you, but I can't quite make out what you have from these photos. Would it be possible to see a picture of it removed from the window? I specifically need to see it from the front and side. Think of it like a mugshot. That will allow me to narrow down the list of possibilities for you. 

A quick learner from Las Cruces New Mexico says:
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Tom from SWISCO responded:

Thanks! Your given dimensions appear to be closest to our 18-009, but you should still review the rest of our measurements to confirm.

Detail of 18-009
Detail of 18-009

Side view of 18-009
Side view of 18-009

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How to Replace a Top Sash Guide [1080p]
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