My first bike at 16 years was a BSA flat head with a chair. 10" was almost standard at that time and has stuck with me through the years even on most of the race rigs I have built.
In the 50's UK working families very rarely owned cars: bicycles, bus, train, walk and motorcycle. Women enters the scene, fine with a dual seat, followed by a child add a sidecar. This changed drastically when the MINI arrived, cost around the same as a bigger bike and new sidecar. The standard lower cost chair was a single seat with a hinged top half, even BSA made one. More than one child get a Busmar double adult with a full door. top left hinged top, passenger cannot get in or out without outside help. lowerright full (home made) double adult with full door, side window and sun roof. lower left my race triton with a hauler platform to carry my 175 Fomula Ducati , (uncoupled at the track) same set up with a trade box on the chair I rode up to Canada with my belongings. Raced the floowing spring at the Daytona FIM, finished 10th. Top right Chris Vincent on the 500 BSA at Governer's Bridge on the way to a win.