Shown above are a set of the famous "Bool" hub caps made by Kent Metal Products for aftermarket replacement on the Model A Ford. These are the 1928-29 style, but the 1930-31 style were also produced.
These inexpensive replacement caps were made and sold through outlets like Pep Boys, Warshawsky, and J.C. Whitney in the 1950s through 1970s.
The "Bool" name in the style of the Ford script was used as way to avoid Ford trademark infringement, yet provide a reasonable appearance replacement 'gyp' part!
Typically they are nickel or chrome plated steel. Used examples are often found in swap meet boxes, but unused originals and sets like this are more difficult to find.