How did these back-ups come to be? Why did this particular short wind up in the 100th anniversary issue of Marvel Tales--a Spider-Man reprint mag? For the answers to these questions, who better to answer than the editor of Marvel Tales #100 (November 1978), Roger Stern? (Lifted verbatim from Sterno's message board, natch!)
RS: Back around 1976, Marvel editorial started commissioning a number of short back-up-sized stories -- partly to audition new talent and partly to help ease some deadline crunches. Scott, Mike (Nasser), and Terry (Austin) had produced that Hawkeye and Two-Gun story and it was sitting on the shelf, when the decision came down in 1978 to make MARVEL TALES #100 a double-sized issue. I had more or less in been charge of the reprint line when I first came to work at Marvel, and at that time I was still overseeing MARVEL TALES.
Anyway, I decided to plug the story in there, before it became too dated. Plus, I really liked the idea of publishing a new story in what was otherwise a reprint comic. I remember it as being a nice little story, and placing it there helped make the hundredth issue of MARVEL TALES something special.
Cool, huh? What? Oh, ya wanna read the Hawkeye/Two-Gun Kid tale? Well, here ya go!