Most likely a studio or pre-production/test fastback build. Noted ribbed reinforcements in the fold-down that I've never seen before. Couple other early cues, like straight handle shifter and square corner sill plates. Also looks like a cloth trunk mat and plate light wiring routed to other side.
Later, the shot of the 289 hi-po emblem is the tall-style, which doesn't appear to have made it into Mustang production.