I agree with what you're noticing Frank.One back that is not as rare as the ones mentioned,but I feel is underrated, is El Principe De Gales.
I've noticed on a few occasions when collectors who have completed (or came close to completing) the T206 set,and included how many of each back they've acquired,it seems as though the EPDG #'s are quite low.
I understand EPDG is somewhere in the middle of the list of tough backs,and it's not all that hard to find an EPDG back for a decent price--but I bet if you tried to do an EPDG only backs set,you wouldn't have an easy time!!!
And I don't understand what's happening with the Piedmont Factory 42's.Are collectors losing a little interest in the Piedmont 42's?
Clayton