A discussion on these popped up in the Blackless thread. There may have been a prior thread awhile back on these
The 1990 set of 72 is the most common and easiest to assemble. It involves two mini cards ( paper versions) of each player in the set back to back in an upright plastic stand ( like mini PSA holders) . One image was from the player's Topps rookie card and the other a mini version of his 1990 card.
The 1989 set of 24 is ( or was) also easy to assemble, but tougher than the 90 set. Same format, rookie card and mini 1989 card.
But as pointed out in the Blackless thread there are two very tough sets of these to assemble. One is the 1989 Mets/Yankees issue of 24 ( 13 Yanks/11 Mets) issued only regionally.
All of these came in solid paper wrappers with related checklists on them
But the hardest issue to assemble is the 1989 Mets/Yankee Prototype or Proof set of 8. Unlike the regular Yankee/Mets issue they have the rookie card paired with the player's 88 card as opposed to his 89 card. Very tough set. Also, they came in different clear plastic wrappers with the checklist
