I agree that it's an Ariel twin-port head with the enclosed valve gear. Getting everything like
pushrods and pushrod tubes to line up, let alone the head bolts must have been quite a challenge.
I know Ariel increased the size of the finning on heads and barrels in, I think, the early 50s but i don't
know if there were any corresponding twin-port heads with bigger fins made. Bigger fins on the head
would have made a good job even better.
Incidentally, according to its engine number the JAP engine would have had a twin-port head originally.