4 KudoZ points awarded for this answer. Comment: "Thanks JS and Ingeborg. I looked for ‘ball lock’ myself but could find no good hits. The Google references to and images of ‘Kugelverschluss’ seemed to have a jewellery theme. Your first ref is good, JS, but the second is a ‘.de’ site and the third concerns tow balls for trailers. Still, one good hit is probably enough and three peer agrees are strong persuaders. Thanks again to both of you."