All SeaCrafts have glass stringers and foam flotation except for the very first one, the Moesly 21, and it has so much glass over the stringers that Moesly said it wouldn't matter if the wood all rotted out because the glass provides so much strength! Not sure exactly when they started using foam flotation; must have been sometime in the late 60's. Two friends of mine had Moesly 21's, a '65 and a '67, and neither of them had any foam in them. (And a tremendous amount of below deck storage as a result!) A 1970 ad for the Seafari 20 on Carla's Moesly SeaCraft site,
http://www.moeslyseacraft.com/upload...boating-ad.jpg mentions the use of spray-in foam and is the first mention I've seen of foam being used in SeaCrafts.
The Specification section on the second page of the attached 1973 Boating Magazine Test of the Sceptre/Tsunami contains lots of details on the laminate schedule and hull thickness!