I believe it's a Top Sash Guide

Hello,

The 18-009 is often suggested as a replacement for your Milgard top sash guide. As long as the rest of the dimensions match up, with the sole exception of the "trim" on the top end of the guide, you'll just need to cut the top down with a utility knife in order to get the clearance necessary for installation. Make sure to wear safety gloves when you do this, just in case. Note that products cannot be returned for a refund after being altered in this way. See our return policy here for more information. Thanks for posting! 

Detail of 18-009

Side view of 18-009