Note the rounded down tube...above 1936 only.
Close-up on the 1936 and up neck and you can just see at the bottom of the foto the "telescopic" down tubes of the forks, 1 tube into another. and the exposed rocker assemblies of 1936 and 1937.
Another close up on the 1936 front head, So when you find them you know what you are looking at,... Had to pull the engine , machine the oil pump surface, we think there may have been a air leak at the interface of the oil pump n case surface... never ever encountered this, and do a proper case fit as the cases leaked, plenty of work , but do it right once and you can put plenty of miles on a properly built engine.
Nice to work on my own stuff for a change.