That was an odd set. Only a few of the cards were packaged as inserts into UD Baseball as inserts - serving as advertisement for their PlayBall game. Those cards have an additional banner above the player's name. The rest of the set came from the game's packs or starter sets. On the back, there's a white number in the lower left corner, above the copyright bar. That number seems to be the original "sequencing" number for the card set, before the typical trading card numbers were assigned.
I tried to create a spreadsheet which paired up those two numbers & noticed some that were switched, or missing. I couldn't track down all of them, but I too added some missing cards to the list, last summer. Just let Admin know & it'll get updated.