It's my understanding that a muffler was optional depending on what state your car was destined for. I run with just the resonator, no muffler and it sounds great. A little loud, but not too loud at all.
I've also heard that some of the early model Formula S's came without a front muffler, but that was changed due to noise regulations. In my 65 Parts Book it lists 2 different mufflers used with the "1-4 Barrel Carb w/Power Pack" engine: 2660 541 which is the Aluminized muffler and 2660 548 which is the Zinc coated version and the "Rear Muffler" is 2660 445. None of these cross over to any other muffler in the 65 model line-up. Never-the-less, it's one sweet sounding exhaust for only being a single.
