68 Barracuda Formula S 340 fastback

See the where the arrow is pointing on the 256 shoe; on the 288 frame it becomes a flat tab. Slides better against the backing plate.
1707782969399.png

Its in the what's new for '68 MTSC
https://www.web.imperialclub.info/Repair/Lit/Master/242/Page13.htm
https://www.web.imperialclub.info/Repair/Lit/Master/