mk II denotes "Mark 2" . This means it is an updated version of an earlier model. For Example there is a C&C 27, a venerable boat of the '70's. There is also a C&C 27 mk II which is an updated 80's version and a very different boat. Same for Catalina 34 and Catalina 34 mk II. Comparing between version is like apples to oranges in terms of price, layout, weight, rig etc.