While I have always enjoyed the design on 1991 Topps, I learned a lot more about the set from David Wright, when he appeared on The Fatpacks Podcast in Episode 1-12. In that episode, David (not the Mets David Wright but the @longflyball David Wright on Twitter) talked about all of the hidden variations from the set; the most interesting of which was the existence of “Glow Backs.” Somewhere in the Topps production facility, there were apparently a few batches of ink that made it onto the 191 Topps release that made the backs of the cards glow under a black light. You read that right, the backs of some of the 1991 Topps Baseball cards glow in a bright pink under a black light.
When it comes to these “Glow Backs,” they are not all created equal. There appears to be a “Glow Back” variation for each card in the set, based on my research. But along with the “Glow Backs,” there are also BOLD backs where the 40th Anniversary logo is bright and bold, and others where the logo is barely visible.