A local photographer is selling some items and has this listed... 1 - RB67 Poloraid filmback HP701 - $15 (1) Will that work on a Pro S? (2) Would I need a 'P' adapter and just what is a HP 701? (3) Is it worth $15? I have never heard of a HP 701 and I have heard some backs don't need a P adapter. (I may have heard wrong.) Thanks. Ric.