I have a small 12.5' fiberglass catboat I'm restoring called a Beetle Boat Swan. It's a fiberglass version of the Beetle Cat made by Carl Beetle who did a lot of pioneering work with fiberglass. My little boat was made about 1952. It has flotation tanks added to the sides just under the deck, these are original to the boat and filled with foam. The last of the Beetle Boat Swans were made by Marscot. Marscot made some of the early O'Day boats.
Flotation tanks were being put in at least by the late 40's /early 50's because my dad recounts a story of sailing his National One Design (wood boat) with his brother (who was crippled from Polio) on Lake Winnebago in Wisconsin and the transom came partially off while out on the lake. He said they made it back to thier mooring due in part to the flotation tanks built into the boat. My dad has always refered to it as a "grand adventure", his brother (my uncle) who told the same story, in later years told me "it was the time your father tried to kill me". My uncle really loved to sail despite his handicap.
John