The 350-460 factory 42 is the only Piedmont back where every card carries a premium. The premium is probably not in line with their scarcity because the back isn't different enough, but they do carry a premium. The other Piedmont back with a premium are the 350s from the Elite 12, a subject that Ted has covered very well:
http://www.net54baseball.com/showthread.php?t=216276