Prism window part.

Hello! Looks like you need a new pivot shoe. My recommendation is the 15-192. This is a Certainteed brand part but this should work perfectly well in your case. The only issue is that ours has two "nubs" on the front that many customers say obstruct its operation in the track. Therefore you might need to trim or grind them off. Just something to consider.

detail of 15-192