1959. Wikipedia: :Galactic Derelict continues the series’ premise, an encounter between Western heroes and a mysterious alien race that has used time travel to alter Earth. This novel shifts between present day and the time of Folsom Man, some 10,000 years ago."
A lot of it takes place in the West, and also a ruined jungle-city on another world. One line in the wikipedia summary might give us the origin of the cover: "After carrying the men through hyperspace, the ship lands on a planet where faltering robots refuel the ship."
I like the little 1960 Christmas-ornament robot in the back; he seems to be clasping all his hands in the hopes this goes well. Oh I do hope the hose doesn't suck out the heart of this one; business has been just dreadful