I have been trying to upgrade my US collection from used to mint and am still missing several from this time period. I don't want to worry too much about the more expensive items or the early classics and if they have interesting postmarks I will probably display both mint and used. I don't want to provide a list but if one of you were to offer a book of mint stamps from this time period at sensible prices (for both of us, of course) I promise to pick up what I don't have! I also don't care about the condition of the backs of the stamps as long as there are no ink marks of thins. Hinge remnants or marks are fine as well as pencil marks and the condition of the gum doesn't matter. To me the front of the stamp is what's important!
EDIT: I find that I look at my US collection, especially the middle area that I used to consider boring, much more often now that most of them are mint. A lot of those wavy line post marks from that time period were extremely boring!!