For the last 4 or 5 years now, checking the backs of any 1991 upper deck cards is priority when I come across them. Already known variations like Luis Salazar and Milt Thompson’s (circle around i in Louis and bullseye in 86 stats for Milt) are a couple of very scarce varieties that affect the reverse of their 1991 cards.
But for some unknown reason, Upper Deck used up to 4 different holograms for their 1991 issue.
The most common is the repeated pattern of “1991 Upper Deck”. This was used across several 1991 and 1991-92 (in BK and HK’s case) sets that year, due, likely to it’s unspecific-sport aspect.
But some cards have shown up that show several other hologram types:
–1990 Upper Deck Baseball hologram are a pattern of baseballs and “upper deck” repeated. I have seen at least 25% of the set with these holograms, even high number cards debunking my theory that the ’90 holo variations were the first batch of ’91’s due to UD using remaining holograms from the previous year. Below is an example of this type:
Junk Wax Gems 
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