Don't know why M-12's are sleepers. Probably because steel shot was mandated for waterfowl and O&U are coveted by "real" bird hunters and target shooters. The good news is --they are sleepers. I've bought four in just the past year - two 16 ga ( $150 & $200 ) a pristine 20 ga SR for $300 and a 12ga. ftw like new for $225. Three of the four were at gun shops!
Lets keep our mouths shut and keep on pumping.