My guess is gouache (an opaque gum-base watercolor) on illustration board. Definitely not Flexichromes; I've seen movie lobby card flexichromes from the era and they look quite different than the 1953 artworks. The texture of the finished items (I saw some at the National) is all wrong for canvas unless the artist gesso'ed the hell out of it then sanded it, which seems unlikely for hundreds of small pictures, and the backs are wrong for canvas. Plastic would not be a medium of choice for works like this. You can turn out nice stuff on plastic but not with the textures and stokes showing on these cards at the speed needed for doing a few hundred paintings for a commercial production anywhere near as easily as on board. Oils seem unlikely just because of the production requirements--so many pieces churned out quickly for Topps screams water-based paint, not oils requiring time to dry. No, commercial illustrations churned out for a commercial production are most likely gouache on board. And damned nice too. Wish I'd gone harder after one in REA. Oh, well. As for glue on the fronts, perhaps residue from the paste-up during the process? I know I have an original piece of W517 art where the pasted-on card number fell off, leaving a slight residue where the circular number should be:
