My take is that off back has become popular with people doing binder sets and aiming to have them be anything but Piedmont or Sweet Cap. Not too many people want to have the true rare backs in a binder, so I think the off back generally has cards split into three groups without any real groups of the backs beyond "rare": 1) Piedmont/Sweet Cap, 2) Off backs = Tougher than Piedmont/Sweet Cap up to Hindu, 3) Rare backs = Hindu and tougher.
And here are some of mine that would fall into that rare category.
