British built only the 221 1937 type, but the imperial armies also used vast quantities of Canadian and USA 24 stud engines of both sizes...I think Carpenters NOS engines are built form Indian Army stashes of Canadian parts.
From memory, the Briish heads that Joblot imported in huge quantities were all British marked, I think E77 prefixes.
These heads are obviously USA and carry a pure USA prefix, and they are different form the 77 prefix heads that would have come on a 1937 truck...the normal head lacked that firing order box.
The regular 77 heads are common because they were used to replace all the rotted out 1937 78 passenger car heads...
These must surely be later replacement castings, but I have never seen any such aside from the British ones!