Those are some interesting photos. My grandfather grew up around 1900-1910 in Endee, NM working on a ranch there (presumably ND Ranch? He's not around to verify any more). At some point, he moved to a ranch in Montana or North Dakota. WWI interrupted his ranching, and sometime after that he left ranch work and became a typesetter for a newspaper, but he maintained some of his ranching Endee connections for the rest of his life. His life from that period is poorly documented, so it's personally very interesting to see photos from the areas he worked in.
It's said the real end of the "wild west" came with WWI, not at the turn of the century. But as you say, a lot changed around 1900. It's perhaps a little surprising that SASS or NCOWS hasn't developed a special class of shooters representing the 1900-1914 time period. (At first this would seem to break the weapons rules, but I'm sure pre-1900 guns were still used aplenty n that time, and B-Western is a recreation of 1950's fantasy using guns from that period in any case. So I don't think gun rules are the issue.)