Unfortunately there are apparently no real true American bison anymore. A sturdy a while ago showed that all American bison in all of North America have varying degrees of cattle DNA. They’re basically all “beefalo”. They are quite different from cattle in many ways, but they aren’t actually really American bison anymore. Those technically are extinct by objective measures, not all that different than if you breed one dog with a different one, the offspring is neither of the parents and also basically nothing at all until some defining characteristics are identified, reproduced and named at least as a sub-breed.
Do you recall anything about where those herds are, because I’ve only seen research showing that all treated herds have identifiable cattle DNA, i.e., https://www.nature.com/articles/s41598-022-09828-z
I was told when I worked at Philmont Scout Ranch [1] that they had a pure herd.
Other than that, I know Steven Rinella listed a few pure herds in his (excellent) book [2] on the American Buffalo, but I'd have to dig it out to find them for you.
"Wind Cave and Yellowstone National Parks are the only two federal herds to have population sizes large enough for sufficient testing. Both herds show no evidence of cattle introgression."
