From Wikipedia (in part)... Though simply cast, promotional models were exquisitely detailed and proportioned, and by 1960, AMT became the main supplier of the pre-assembled model to American car companies. AMT worked most closely with Ford Motor Company and General Motors Corporation, but promo contracts among the model manufacturers seemed to alternate year to year (See Doty 2009a for a nice history of AMT's Ford Galaxie). Shapes of the vehicles were near perfect, though in the 1950s, cellulose acetate, the plastic of choice, was prone to serious warping. In 1960, AMT and some other manufacturers switched to styrene (the brand name being Cycolac) and by 1964, all of the major model car manufacturers had changed over to the new plastic. This solved the problem and styrene models 50 years later still maintain their form. 1961 was a key year for the switchover. For example some early 1961 Mercury Monterey two door promos were done in acetate while most for that year were done in the new non-warping Cycolac