Just for interest, what came first, the awning, or the awning rail?I guess that it would be the awning, so how did an awning attach without the rail? And how could you get a weatherproof seal?
My guess would be that before awnings small tents were erected at the side of the caravan. Some enterprising manufacturer spotted the gap and came up with the awning rail and sold bespoke awnings as an extra. Like everything else the rest of the market would soon pick up on a good idea and do something similar.