Don't think its JAP, doesn't have any of the typical JAP features.
16H looks to be a different stamping ??
There is nothing (numbers) on the drive side crankcase. ?
Looks to be a Sturmey Archer - made by Raleigh's of Nottingham - and supplied to a large number of makers,
including some in Europe.
Only match can find is a part timing cover view
http://cybermotorcycle.com/gallery/sturmey-archer/images/Sturmey-Archer-1936c-600cc-SV-Ransomes-2.jpg(And I apologise for all that dirt !)
Don't know why your timing cover doesn't have the SA logo.
Can we also see a Sturmey Archer gearbox behind it ?
They usually came paired, although not always.
Sturmey made quite a range of engines, yours looks to be late 1920s or early 1930s.
They stopped making motorcycle stuff in 1932 (was it ?)