The Series 1 combustion chamber design, piston bore size and I think the valves also were 'borrowed' from the Cub flat twin diesel, which was an excellent engine. The transfer was not successful and the Series I had a reputation for being a bit if a b*gger to start, hence the hasty direct injection setup of the Series II engines.
The engines with a Z in front of their engine number are Series II but may be badged or number sequenced as Series I....
Peter