I have 3 Formula S Fastback Barracudas. I've pulled the Numbers from the original rear springs.
1966 Formula S, 4 speed = 2403804 both sides
1967 Formula S, 4 speed = 2835024 both sides
1968 Formula S, Automatic = 2835056 both sides
I also have MOPAR replacement springs for a 71 340 Duster, 4 speed = 3722133 both sides