Window balance cap I believe

Thanks for posting! Looks like you need a new top sash guide. We offer a ton, though, and I'm not sure what to recommend without seeing the original. In order to ID a match, we'll need you to remove an undamaged version--either from the other side of this window or an identical window in the house--and show us a photo of it removed. Try to take your pictures from the front and side, like a mug shot. Dimensions would also be a big help.

In the meantime, the video below should cover everything else you need to know for this project.

Perfect, this is exactly what we needed. Right off the bat I think our 18-166 is a very close match. Look over the measurements we show for it and let me know what you think.