I looked at mine and I "think" it might have a small nub like in the pic below the plug.

the end of the head has a machined flat with the small core plug boss,it also has a threded set screw in the machined flat,looks to be about a 1/4-20 size. not sure what thats for

I see no other markings cast into the end of the head though.

I will pull the plug to confirm it has that nub under the plug

I didnt know a 452 head also used a 13/16 plug with a flat washer either.