The Seal 28 was designed by Angus Primrose and built by John Baker between 1976 and 1982. Sail numbers were from 1 to 89. This is a substantial cruising yacht with masthead rig, lifting ballasted keels; 6 berths with 2 of them in separate aft cabin and a 20 hp diesel, making it also a convincing motor sailer.
The Seal 850 was a development of the Seal 28. It has the same hull and rig as the 28 but a larger saloon and cockpit instead of the aft cabin; 5 berths; reshaped cabin top and transom hung rudder. This gave the 850 a sleeker look and better performance.
Mike Edwards, who owned S28/26 Aztec, has kindly supplied these 2 PDF files:
Seal 28 Plans: Specification & Rigging Dimensions, Original Sail plan, Original mast, New mast and sails, deck layout, Internal layout, Saloon berth layout, Mast support, Mast spreader, Mast head, New Mast head, Mast foot plate, Rudder, Winter cover and Cradle
Seal 28 Projects: Switchboard, mast wiring, water tank, clutch bracket, tiller extension handle, engine & prop shaft, propeller, mooring riser, instrument wiring, Electric keel arrangement, stanchion top, Exhaust water trap, Winter Cockpit cover