My guess is mid '70's too especially if it's a "trihedral" hull.....one large center hull....like today's deep V monohulls, with lifting strakes and a short sponson on either side of that maybe half as deep, or less as the center hull. Idea was to have a nice soft ride from the center hull in rough water but have the side buoyancy of a tri-hull when at rest. As I recall there were 2 basic hull lengths (17 and 19 footers) in the smaller hulls and you had to get at least a 19 footer to get that hull. Jerry Whittle or Phil Dill boats in Lewisville, TX. sold them.
