In the late 40s and onwards (if not earlier), lacquers might have been used for repairs, because they dried faster than the synthetic enamels that were baked on (and/or took longer to dry).
Advance-Design trucks (and maybe earlier trucks) were painted at the factory with "high-luster Dulux synthetic enamel which is baked on in thermostatically controlled ovens." Air-drying Duco lacquer was available for repairs.
The closest type of modern paints would probably be acrylic enamels (with or without a hardener). A 2-part, single-stage urethane enamel would be a very durable modern paint.
DUPONT Duco lacquer DUPONT Dulux alkyd enamel Another posting on Duco & Dulux paints 
