With all due respect, if the '58 yellow letter variation is indeed a "variation", in this set, then the '58 Blue Background Aaron is a "variation" as well! An error card is caused by a group of cards, (more than one) that are the same, but being different than the other cards in a set, caused by either the machine's malfunction, or the human operator's error on a paticular run, on a paticular day, (A dripped "ink spot" does not apply here). This is true with many different collectibles from all over the world, including, mis-stamped coins, errors in printed currency, stamps, cards, and etc.
With the 1958 Topps yellow letter variation, a group of cards had a different color of ink from the standard cards in the set. Also, with the 1958 Topps Aaron card, a group of cards had a different collor of ink from the standard cards in the set. Plain and simple...the 1958 Topps Aaron card with the "Blue Background" is in fact an error card, and a variation within the set! As more collectors share their "Aaron Bluebacks", the industry will in fact one day, recognize this variation, as an error card! I am 54 years old, I hope it happens in my lifetime.
Congrats to those of you who have been lucky enough to find one of these Aaron "Blue Background" cards.
I was lucky enough to secure an OC example about 40 years ago in Atlanta, (shown below). I also saw another one about 10 years ago at a Charlotte card show., but it was very rough. After doing an on line search, and reading these two threads, I know that at least 8 of these cards existed at one time. I'm very sure more are out there although very rare, and I hope people will continue to post them.
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It was never about the bubble gum...