I've heard by way of anecdote some of the connotation was strengthened by the Mattel toy guns being bought and used as training aids by individual units in stateside training. That could strengthen a baseless, but not absurd, rumor that Mattel had anything to do with the polymer on M16s.

Anyways in the movie 'The Green Berets' (1968) John Wayne smashes a Mattel gun on a tree. It isn't a good movie, but it has weirdly memorable scenes.