Window repair

Thanks for posting! The piece you are searching for is the top sash guide. This gets installed at the top left and right corners of the window and is used to guide the balances in the window track and prevent it from shooting into the framing.

Yours reminds me of the 18-161 5/8" Sash Guide. To determine if this is the correct replacement match, carefully compare the dimensions.

Detail of 18-161

Profile view of 18-161