Wen Mac lineage

Sorry if this may be an old subject, but my search came up beans.
I am interested in the Wen Mac engine lineage.By cast model number is my standard.
As best as I can tell, there are EIGHT named variants; of course within each named
variant the are surely un named deviations.
So forgive my lack of knowledge. I started collecting Wen Mac motors that I had as a kid about 2 years ago.
This has been a lot of fun for me, and a rewarding experience. Here is my knowledge of
cast model numbers;

MK 1 (or first model) - Just cast "Wen Mac" on port side. No MK#.
MK II seems to be the most available model. Many variants as I see them.
MK III
MK IV
MK V
MK XI- very elusive. I recall seeing one a few years ago but this is one I am still looking for.
MK XII
MK XIII

Notice lack of MK VI,VII, VIII, IX, X, if my search is accurate. I don't know if Wen Mac produced any
engines bearing these model numbers.