http://bobstory.tripod.com/faq.html#Bare%20Metal2
..."Synthetic Enamel: In the mid 1930's, a new and completely different type of paint was developed, the alkyd or synthetic enamels. It proved to have superior qualities in film strength, adhesion, luster, flexibility and durability over all previous paints. The resin base was developed from the reaction between phthalic anhydride and glycerin, with gums, oils and plasticizers added during the manufacturing process a drying oil such as linseed, a polyhydric alcohol, such as glycerine, and a dibasic acid, such as Phthalic Anhydride. It dries by solvent evaporation, like the lacquer paints, but the resin remains soft and sticky when no solvents are present. It cures to a hard finish by absorption of oxygen from the air. The curing process can be accelerated by heat, and several methods of baking enamel were developed. Unlike lacquer, when dry, it needs no polishing to produce a high luster finish. Acrylic Lacquer: As time passed, chemists developed a substitute for nitrocellulose lacquer, using an acrylic resin as a base. The resins used in acrylic lacquer tend to be slightly brittle. This deficiency is overcome by the use of a plasticizer ( a liquid that is a solvent for these resins and softens them slightly). A cellulosic resin is any resin derived from cellulose (pure cotton). Acrylic lacquer was used extensively by General Motors. Acrylic Enamel: During the late 1960's and early 1970's..."
It seems that "Acrylic lacquer was used extensively by General Motors" might be the answer to the frame paint question... I'm waiting to see what reply's I get to my question in the forums at stovebolt.com