Besides the 1952 Clowns PC, there is another "true" baseball card that is unique...
I dont own it anymore, but this card below is the only known example from this series (set?)
discovered only about 4 years ago....
...maybe go for every baseball card of Hank from 1954-58
PS on the 1958T Aaron with the blue background, Ive seen 3 of them now in last 2 years not including one that was 1/2 green and 1/2 blue. they are interesting but are really a print defect (missing yellow) not a man made error/variation. I believe there are quite a few of them still to be "discovered".... I have a search programed for the '58 Aaron and run into the blue ones from time to time on eBay