Barracuda factory exhaust cut-out ????

Geof...I think you'll find the FS exhaust did have a muffler...albeit, almost straight through...very little muffling...specific to that system. The pic in my post above is the Accurate Ltd repro exhaust system for the early FS cars. It is a direct copy of the original systems, complete with all components.

The Commando exhaust included a muffler- owned first '65 in 1966.
Also because it had a 273 Commando didn't mean it was a Formula S. You could buy a "Rallye Pack" car which had only the motor and exhaust- no sway bar, tach, and came with 13 inch tires/wheels----