I did a post about them on my blog last year, here are a couple of paragraphs that covers the history of the Exhibits sets:
"The set is based on the cards that the Exhibit Supply Company produced between 1921 through 1980. They produced a set nearly every year, or multi-year sets, until 1966 and then one set in 1977 and a final set in 1980. The Exhibits insert is based on the cards between 1921 and 1966.
The checklist contains 50 players and is made up of current (for 2004) and retired players but the kicker is that each player has at least 45 variations with AL players having a 46th variation. To add to the difficulty is the cards are limited to the year the original cards were released meaning that the cards based on the 1921 release are #/21 through the cards based on the 1966 release, which are #/66. If that was not difficult enough there are variations of the variations, I know… we are getting in to Panini realm now. For example, there are cards based on the 1963 set (so they are #/63) but then there is a card with no name on the left, one with no name on the right, one with the name in black on the left, one with the name in black on the right, one with the name in red on the left and one with the name in red on the right."