...Boss of the Plains with a 7" crown? I've never seen anything in pictures...
I'm new to this forum, and this is my first post here.
I can't say anything about the "Boss of the Plains" as a specific Stetson model, and the photos I'm thinking of may well come from 10 to 20 years later than the above referenced document covers. But I have seen photos of 7" open crowned hats resembling the "Boss of the Plains." I have seen them in several of Edward Curtis' photos of native Americans he travelled with. One such photo, (page 101, "Edward S. Curtis - Coming to Light", by Anne Makepeace, National Geographic Society, 2002) shows Curtis with Goes Ahead, Hairy Moccasin, and White Man Runs Him. While there are decided problems with the "props" associated with Curtis' portraits of native Americans, this one is a relatively candid photo (it's whole style is different from Curtis' fine art photos), posed but in the middle of the field somewhere - everyone seated on the ground, horses in the background, notihing but big sky and low hills as far as the eye can see. It appears to be an honest photo of the group while travelling. As such, I think it far less likely Curtis added props to this photo. Both Hairy Moccasin, and White Man Runs Him are wearing tall crowned "Boss of the Plains" style hats that are probably around 7".
Another photo (frontispiece, titled elsewhere, "Curtis and friends doing fieldwork," shows Curtis with an unidentified group of native Americans, three of which have these same shape tall open crown hats. This photo is more typical of Curtis, clearly posed, clearly carefully shaped (and photographically beautiful) composition, so clearly intended as fine art and perhaps props are a possible problem. But the hats are in the photo, and so whether props or no, they existed at least by the turn of the century.
(I cannot find these photos online, so haven't provided links. Sorry.)
Whether they were often associated with cowboys, or whether they were associated with cowboys during this period is another question. Maybe Rickey got it right, or maybe he misidentified the dates of some artifacts? I can't say, but the approx. 7" open crown hats existed historically, and were worn at least by some historic native American individuals. (The three scouts mentioned above rode with Custer's 7th at the Little Big Horn, and are said to have warned Custer the Sioux there were more numerous than his army's bullets.)
Just thought I'd pass the info along. Up to you to decide if it's close enough to "period correct" to help anything.