On this white center cap which I purchased seperately from my car you can see the original Mustang part number cast into the back of the part which is F1ZC-1A096-AA (F1 is 1991, the first year pony 5 spoke wheels were offered) as well as a sticker showing part number F3ZC-1A096-BA (which I assume is the revised number for the part finished in white for the feature Mustang). These did not come in the original boxes.
The first pictures are the center caps from my car (which are original) and have the identical sticker with the same part number on the back.
The last three pictures are those that Paul sent from the set of caps he purchased for his Mustang. They are the same caps as mine but the part numbers are slightly different. The number on the box is F3ZZ-1130-B. His also have the original number F1ZC-1A096-AA cast into the back. The box indicates what may be a date: 10-25-00