In 2004, Upper Deck, among other companies still had a license for making cards and today's set is Upper Deck Vintage. The set consisted of a total of 500 cards, broken down into 300 base and 150 cards short printed across seven subsets. I say subsets instead of inserts because they are continuously numbered to 500.
Mark Prior was coming off an All-Star season that saw him come in third in Cy Young voting (behind Eric Gagne and Jason Schmidt). This particular card is from the Play Ball Preview subset (301-315).
I'm not sure how this a preview because all of the Play Ball cards from 2004 I've seen, look like this:
Maybe since its from 2004, its a preview of 2005? Well, I couldn't find anything about 2005 Play Ball, so I think they were a one and done set.
The first 350 cards in Upper Deck Vintage year (which included the 15-card Play Ball Preview set) also had black and white parallel versions.
The backs are identical so the only difference is that the front is entirely black and white.