Plastic Cap - Top of window

This part is known as a top sash guide. We should be able to help, but I need to see a picture of this part removed from the window. I will also need dimensions. Check out the video below for more information on this kind of issue.

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The top sash guides look like SWISCO product code 18-042.

It also looks like the take out clip is missing, you will need these to remove the window sash. It is most likely 16-002 or 16-023. See if you can find one in another window to compare it.

The top balance fitting looks like 19-018; if you need to replace the entire balance see Series 377. Your balance is marked 26 4, check the length (exclude the fittings for this measure) it should measure 27", if it does go to part number S377-27 and select stamp option 2640.