The player deals seemed inadequate to (MLBPA Executive Director Marvin) Miller, who set up a meeting with Topps, whose president, Joel Shorin, told him: “There will be no changes because, honestly, I don’t see the muscle in your position.” This response did not surprise Miller – he knew that he was not going to get a better deal from Topps by appealing to Shorin’s sense of fairness. That is not how labor battles were won. He needed muscle.
In early 1967 Miller suggested to the players that they stop renewing their individual Topps contracts and boycott Topps photographers. This was the only way, Miller advised, that they could get Topps to deal with them. Although the action was voluntary, Topps was able to take no more than a handful of photos during the 1967 season, and, with the dispute unresolved, none at all in 1968. This had an effect on the 1968 Topps set, which was not able to show as many properly attired photos as usual, and a much more dramatic effect on the 1969 set.
Source: Mark Armour, SABR Baseball Card Blog