This is a very interesting book. It has not only great Jack Kirby art but also a good story, much better than typical Stan Lee & Co. Atlas westerns. Whip McCall's ambiguity adds a bit of depth to the tale. The writer makes good use of all 26 pages. He give us some nice cinematic moments like McCall's surprise death. I'm no expert but I have the strong impression that Kirby wrote this as well as pencilling it.
After reading the links to discussions about the strip's origins I suspect it was an unpublished story produced by Atlas/Marvel for whatever reason (a proposed one-shot, maybe?), that was lying around the Marvel archives. British publisher Alan Class described buying the material in his comics from a middleman who packaged stuff from American publishers for sale overseas. It came in the form of a random stack of photostats from which Class picked what he wanted to print. The packages were a hodgepodge of stuff from different publishers: Marvel, Atlas, ACG, Tower, even some Golden Age leftovers.
Comics researchers have demonstrated that artwork on foreign Marvel covers were sometimes earlier versions of the covers of the American editions. At other times they were art that was unpublished in the USA. I speculate that "Showdown at Snake River" happened to be in one of these export packages which found its way to Italy and Australia. That's my guess anyway.
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Showdown at Snake River