i've had virgin 68/9 318 2bbl and 340's open and the steel shim is unique to the 273
ETA: i should've been more pointed. i *know* the steel shim with the unique shape is specific to the commando motor. i'm unsure on the 273 2bbl motor in that NOS gaskets i've come across are composite. however, when i tear down the unsullied 273 in the 68 dart i'll report back.
more to the point though, the OP's gasket looks like the fire ring is a big larger so i'm *guessing* it was open at some point. it seems reasonable that somebody would just knock on a set of 318 gaskets because they were widely available probably cheaper and they likely wanted to lower the compression slightly to be more gas friendly.
although, that's mostly a somewhat educated three raccoons in a trench coat guess. do with it what you will.