Looking for a children’s picture book, probably mid- to late-1950s or early 1960s. Size of a Golden book, but not that publisher. There were large-ish colored triangles on top corners of each page (maybe bottoms, too). May have had an illustration of a little child dressed up as a cowboy; I seem to remember something about a lasso. Maybe a lasso acted as the frame of a page or pages as well. Bright, cheerful colors.