Cool video of 68 barracuda changing from dual to single exhaust like the 273 commando

66fs said
The early systems had a 2 1/2 straight through muffler, followed by the 2 1/4 straight thru resonator. It is a nice alternative to duals if you lack space or want to simplify your system.

Here's a little info on that straight through muffler. Commando Exhaust