Most readers of "Adult" westerns assume the concept was born back in the 70s with series like Slocum (think about it) and Longarm, the guy with the long erect cigar between his teeth. Truth is, they were around as early as 1936, when Spicy Western Stories was born. And Robert Leslie Bellem, creator of Dan Turner, Hollywood Detective, was one of the mainstays of the field. "Lust of the Lawless" is from the eighth issue, of June 1937.
A note about the cover: "Lust of the Lawless" was not actually the cover story. That honor went to "Hero of Coffin Creek", one of the long-running series of Buckner Grimes stories by Robert E. Howard's pal E. Hoffman Price (and obviously inspired by, but vastly inferior to, REH's Breckenridge Elkins tales). Price's stories are not bad (well, not really really bad), but Bellem's are usually better.
Unless you're a world-class squinter, the best way to read these pages is probably to right click on each and open it larger-than-life-size in a new tab. Have a Spicy day.