Authority or not, thanks for that link! Seems there was indeed a specific ensign indicating a privateer sailing right toward your own ship, with intent to, umm, privatise? I'll have to study it further, including all zero external links, but it seems to confirm my suspicions from childhood (I haven't thought much about signal flags for quite some years now). You have referenced red jacks and black jacks. Now I'm wondering whether the playing card or the banner came first.