|The freeboard of the Mk 3 is 100mm higher than the earlier models. They are all swing keels, its the modification to drop keels that often confuses people where a low freeboard Mk1 or 2 has a drop keel and could be mistaken for a Mk4. |
Look at the photos and you should see the different proportions of an early and late hull.
Also Mk3s have swept back rigs generally without backstays. Mk 4s have full battened mains with lots of roach like a catamaran.