I too agree that they should be higher on the difficult scale. I haven't been following the Group of 12 cards in a few years but at one point, it was a focus of my T206 collecting for a few months. I managed to find Crandall, Duffy, and Sheckard out of the 12. I admit, I haven't focused my collecting on Cycle 460's at all but have 25 different in my collection. Here is my lowest combined (PSA + SGC) population card of 2 total.
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Ron - Uncle Nacki
T206 Master Monster Front/Back Set Collector - www.youtube.com/unclenacki
T206 Basic "The Monster" Set
514/524
T206 Advanced "Master Monster" Front/Back Set
??
??/5258
COMPLETE T206 BACK SUBSETS
Old Mill Southern Leagues - Black Ink
48/48
Sweet Caporal 350-460 Factory 30 Full Color "No Prints"
28/28
NEAR COMPLETE T206 BACK SUBSETS
Polar Bear
245/250
Sovereign 460
50/52
Sweet Caporal 150 Factory 649 Overprint
31/34
Piedmont 350 "Elite 11"
9/11