I bought this for a few reasons: It's attractive, I happen to browse the SB auction at the right time, and 1483 is a prime number. The next newest coin in that set is a 1693 British shilling, and I don't expect that filling in all the blanks from 1483 to then will be easy, but I just had to have this.
The piece has great eye appeal, but.... what is it? Could you describe it for those of us not in the know? Is the date actually on the piece? (I don't see it, but I may be missing it). That seems earlier than most pieces usually had dates. I really like the idea of your prime set, it is a really cool set.
It's dated 1483 in Roman numerals on the inner ring on the reverse (MCCCLXXXIII starting at about 8:30). It is a Bausche of Wilhelm IV of the Jülich-Berg Duchy. It's about the diameter of a quarter, but thinner. A little smaller than an English groat.
I figured it was Germanic of some sort, based on the style and script. Found a Wikipedia page about the Duchy: https://en.wikipedia.org/wiki/United_Duchies_of_Jülich-Cleves-Berg Cool stuff! Do you have a Registry set, or photo album somewhere, where we can see the rest of your set?
As I noted in the "Unexpected Purchases 2018" topic, this was a pleasant surprise and I wish you well with the project.