"Butted" means the seat tube was single butted-- butted only at the bottom; the top tube and down tube were butted at both ends, hence "double butted."
My Claud Butler Olympic Sprint is apparently straight gauge 531, but throughout. These frames were offered in both butted and plain gauge versions. Being a repaint, I don't know what the original decal said. Other examples of this model that I've seen in photos had a variety of Reynolds decals, including the usual "guaranteed built with Reynolds 531 butted tubes forks & stays" one as well as "Reynolds 531 frame tubes" and "guaranteed built with Reynolds 531 plain gauge tubes forks & stays."
snip . . .