MARK I       Dog house                                                                                     MARK II Flush deck
 

Mark I SJ21s have a Dog House deck lay out with the deck line dropping down to the sheer level forward of the mast, while the MK II SJ21s have a
Flush deck which is rasied all the way forward to the bow. The MKI has no aft storage in the cockpit while the MK II does. The MK III is just like the
MK II with a changed interior.