About six months ago, I posted a PDF of an envelope that used this illustration, which is based on a 1930 painting by Maurice Logan, saying I had the envelope but no letterhead. It turns out I do have the letterhead, and here it is.
I have no idea when SP might have used this letterhead. Certainly it is after 1930, when the painting was made, and before 1960, when SP was into much more modern designs. I’d rank it more modern than the Apache letterhead, which was from before 1938, but older than the monochrome letterheads used for the Argonaut, Cascade, and other trains, which I suspect date from the 1950s. That suggests this letterhead is from the 1940s.