Yours is an interesting review. I have read perhaps sixty L'Amour novels so far, including all of the Sackett books. I guess I have become a little jaded, since so many of LL's characters seem to be cut from the same bolt of cloth, or forged from the same roll of steel. I did not come to the conclusion that Passin' Through is a Sackett, but I can see that it is possible.
I immediately caught the reference to their arch enemies - Mattie is a down-on-her luck Higgins gal - but I don't necessarily see that as an indicator that the main character is a Sackett.....
It has been eight years since you wrote this review, but I wonder if you can comment about what made you conclude he was directly related to those hard to kill, fast shooting boys from Tennessee.
BTW, I enjoyed this book _immensely_, and I agree that it is among his very best works. That part of the country south of the San Juan Mountains near Mesa Verde in Colorado is drop dead gorgeous, and I kept stopping reading to Google pictures of the places he mentions.... Search for "Maggie Rock Ranch" if you want an idea of what this country looks like.
Spokane WA USA