Harley Davidson servicars, seemed to be attaching to cars with a normal trailer hook, maybe with a c-clamp on the bumpers?

whatever it was, the photographer seemed to be focused on taking the photo of each of these guys at exactly that moment

http://www.sandiegohistory.org/collections/motorcycles/motorcyclese.htm

Update, Thomas at the garage journal thread about Johnson's service station has found a bumper clamp mount for towing from a tow hitch, to the car you are pulling, by means of that follower car having a clamp on it's bumper

http://www.garagejournal.com/forum/showthread.php?t=51567&page=346