This Universal Engine History
is the web site I was referring to in my original post. It says "In 1977 the Model 5411 was introduced. It was a 31 cu in, 2 cylinder raw water cooled diesel producing 11 hp @ ? rpm, based on the Kubota Z-500 block, and fitted with a Hurth HBW-50 marine transmission." and "In 1982 the Model 5411 was replaced by the Model M15, a 45 cu in, 16 hp, 2 cylinder, fresh-water-cooled, based on the Kubota Z-500 block and fitted with a Hurth HBW-50 marine transmission." Is this web site in error?