The Mark J can be identified by a serial number stamped on the end of the crankshaft, and on the flywheel near the SPARK indicator. My engine only has the crankshaft number, and that was quite difficult to find through the rust and slap-dash method of stamping. My engine also has 24 cooling fins on the cylinder, which indicates a Southern Cross.
This number is a bit difficult to read, but polishing the steel with fine sandpaper brought out the parts of the numbers which were actually stamped. The serial number 5853 places this engine at 1938.
Southern Cross Engine Register maintained by Ian Mathews.
Michael Pedersen's Buzzacott A very good writeup on the Buzzacot version of this engine.