273 Commando or Not?

Welcome to FABO! All Formula S Barracudas in '65 came with a 273 Commando engine. If you're curious as to whether yours is original and commando, put it on a dyno and see what the horsepower is compared with the specs for a 2bbl or measure the cam lift at the rocker. Short of pulling a head, I don't know any other way. There is nothing stamped on the outside to designate a 4bbl engine. If the engine has been rebuilt sometime in it's life, it could have anything internally. If you're going to rebuild the engine, it really doesn't matter what it has now. Both engines used the same block, head, crank, etc, castings/forgings. Build it to run on the fuel you choose to use and the purpose for which you will put it. There are no numbers matching drivetrains in those years so the best you can do is get casting dates 2 weeks to 4 months before your production date.