Hi Justin, thanks for your answer.
Notice that the engine is not connected directly to the frame, but it has that black metal plate visible on one of the pictures. So the frame and the engine comes out of different motorcycles.
Amal Carb. has a lot of stamps on its case, logos etc. Engine has none, except for a serial number. It is not german-style, they always has everything well documented and stamped I think.
What do you think?