There were also tandem hubs that had threads on both sides in order to mount a brake on the left side. Somebody used to make a thread-on drum brake and I think there were disc brakes, too. They weren't meant to stop the bike, but were used as an assist to the normal brakes for long decents so the rims wouldn't get over heated. Some people would use a single friction shift lever to set the drag, then modulate with the rim brakes. Others would mount a brake lever on the stoker's handlebars for the hub brake.