The heads I currently have on my 318 are casting number 2843675. This number is from the bottom of the head,same side as the valve face, not under the valve cover, where the valve stems are.
I say it like that because the 273 heads I have don't have any numbers except in the valve stem area under the valve covers. I have a set from a 1967 273 and and a 1966 273, both not commando engines, and I checked a head from each.
EDIT....
OK! FABO ROCKS! I found a post that lists some casting numbers. The 273 heads are 2656920.
http://www.forabodiesonly.com/mopar/showthread.php?t=249026&highlight=273+casting