Nice find. That style wheel cover was offered by Dodge, Plymouth, Ford, and Chevrolet and were all made by the same manufacturer that supplied the wire wheel covers for most of the US auto manufacturers in the mid to late 60's. Dodge and Plymouth versions were 14" only and had Dodge or Plymouth emblems in the center. Chevrolet's versions were made in 13" and 14" sizes and had the Chevrolet emblem in the center. Ford's version of the same wheel cover was offered in 14" and 15" sizes and came with either the Ford emblem, or the Cobra emblem (on 68 Shelby Mustangs) or came with the plain center. The advantage of the plain center ones is that they are identical to the ones used by all the other manufacturers but with the plain center, they can be used on any Plymouth, Dodge, Chevy or Ford and look correct. I believe the plain center 15" were pretty rare and may have only been used as rarely seen options on the 68-69 Ford Galaxie XL (and you are probably right about the Ford pickup application as well). They look right at home on a Plymouth as well.
