One of the rules referred to above applies to 350-460 subjects (excluding the six superprints).
If found with a Broad Leaf 460 back, it should also be found with a Red Hindu back, and vice versa. But if found with either of these backs, it shouldn't be found with either an American Beauty 460 back or an Uzit back.
This mutuality / exclusivity also works the other way around, too.
If found with an AB460 back, it should also be found with an Uzit back, and vice versa. But if found with either of these backs, it shouldn't be found with either an BL460 back or a Red Hindu back.
Since, if memory serves me, there are 56 non-superprint 350-460 subjects, 112 possible backs should automatically be eliminated.
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A couple years ago I put together a Polar Bear subset.
Cards with Polar Bear backs can be found for the 350-Only, 350-460, 350-460 superprints, and 460-Only subjects. I think if you total up all these cards, it comes out to be about 317 that could potentially have Polar Bear backs.
However, I only found 250 cards with Polar Bear backs. Surveys ran by Ted Z and others, along with searches I've done, have failed to uncover any additional cards with Polar Bear backs. So, there's most likely 67 Polar Bear no-prints.
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For the Piedmont 350-460 Factory 42 backs, all six 350-460 superprints have been found with this back, some of the 460-Only subjects have been found with this back, and all but one 350-460 subject have been found with the P42 back. The one 350-460 subject that has never been found with this back is Tinker Bat Off. I strongly believe that it will one day turn up with a P42 back. Of course, it could also be one of those weird rule breakers and was never printed with a P42 back.
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The examples above are just the tip of the iceberg when it comes to determining how many different T206 front/back combinations exist.
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Finally, when looking at surveys, keep in mind that they were done by humans and are susceptible to human error. Consider how easy it would be for someone to accidentally report that they have a Harry Davis, Davis on Front with a Sweet Cap 350 Factory 25 reverse, when they really meant Harry Davis,
H. Davis on Front.
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So how many T206 cards are in a master set? Well, I'm up to 1512 different front/back combos, so I know there's at least that many
Best Regards and Happy Collecting
Craig