I have had SOS with all kinds of meat. My understanding was it was any meat sliced, ground, or shredded in a gravy over any type of toasted bread. The way my mom made it growing up was with chipped salted beef in a jar with a simple cream gravy over toasted english muffins. We called it “cream chipped beef on toast” but mom admitted later that she learned it as S.O.S. Or “shit on a shingle” . Ive had it that way, with ground beef, turkey, and roast beef. All of them delicious.