Thanks Brian,
It is the straight back crankcase.
The Log book frame number is 1538. (no “M”) but the SOC had it listed as 1538M.
A close examination of the number on the frame reveals two marks that could be from the top of an M stamp that was badly positioned when struck. I will have to check this again!
It seems to me that if these “AM” engine numbers were previously unknown, they must be quite rare and somehow special?
