Need help identifying part

Hi. I need to know what the part is in the top corners of this bottom sash of a single hung window, as the piece on the opposite side broke and needs to be replaced. Unfortunately there is absolutely no marking, name, or numbers on the windows, the frame, or the piece itself to ID the manufacturer. The windows are likely 20+ years old (former-former owners had them installed, I think). (please pardon the dust!) Hope you can help. Thank you!