The heavenly twins has been produced since 1971 in the following variants (major changes/distinguishing features are indicated):
Mk1 - shorter keels and no central nacelle/pod aft of the cockpit, rounded saloon access door
Mk2 - As Mk1 but larger rig, square saloon access door
Mk2A - New Hull mould, incorporating longer keels (fwd) and full length central nacelle
Mk3 - Same hull as MK2A but with new Deck Moulding, incorporating larger aft cabin hatches, additional re-enforcement under mast/above saloon door
Mk4 - Solid foredeck, improvements inside.
HT27 - Hull/Deck join lifted 100mm, giving height for a "stoop-thru" passage in stbd hull, deck mould altered beneath mast to give standing headroom in saloon