With the information I posted with the letter from August 1960 for the first brochure, it mentioned that the three gearbox models were not yet available but would be in production soon, so that sort of dates your very early gearbox engine to sometime between the end of 1960 and early 1961, the next set of parts diagrams date from August 1961 (only two piston rings were used by then).
And looking at the gears fitted, this engine looks to be a Model H (1700 R.P.M. with clutch).
David