Looking at the contemporary drawings (and definitive data is a bit thin on the ground) One (earlier I think) version has a vertically lifting keel and a two-part rudder with a GRP shell into which a "boiler-plate" blade retracted when persuaded by a line led up through the pivot tube to the tiller.
The later(?) version has a bigger, deeper fixed keel with a slightly narrower slightly swept-back drop keel. This obviated the need for the problematic lifting rudder blade and the rudder is one piece and supported on a longer skeg nearly as long as on the fixed-keel boats.
I have JB drawings of the 2 versions from his published literature but the won't fit in my scanner and I'll need to wait for daytime to use my camera. Then I'll have to relearn the photo-posting procedure!
Incidentally I've never found any mention of Mk I or Mk II in the original stuff and his sales leaflet always showed a side view of the fixed-keel version.