I've got it home... its an April '53 engine so a bit early as mine has a September engine, but I also have a september engine too... so maybe I can make one from the pair?
This one was not sceized, but would only do 3/4 of a turn...
So i pulled off the offending head...


oops...
The weird thing is that the head has no markings...

Repro? Aftermarket? :?
The one on the other side has this marking -

The september engine has "proper" marking son the heads -

Can anyone tell me why the april engine has longer sleves on the barrels than the september one?

(I also have a rebuilt march '53 engine with the longer sleves too from my Irish car)