Top Sash window guide

Based on your photos, I believe you have either our 18-060 or 18-161 top sash guide. Look these over and see if either one is a good match.

You may need to remove the original to get the best comparison. If you do, just be careful when you remove the screw, as it may be holding the corner of the window together. You'll have to hole it together by hand as you remove them just to be safe.