Joe,
No I have never come across a HA Wallis that I know of, until seeing Graham's 25 with HA stamped in the engine casting I would not thought of looking there, I would of expected it to be stamped after the serial number on the oval plate, so this proves we need to spread our vision wider, we have been missing vital clues over the years.
Keep digging in the parts books, the aluminum head is something else I have never heard of on a M-H tractor, I wonder if they warped when run hot, some cars over here had the same problem years ago. especially the air cooled Hillman Imp, they just could not keep aluminum heads sealed on them.
Malcolm.