Hello All This is an interesting subject. I have had several of these bike through my hands, all BSA's. Over here many just call them BSA folders-- because they fold, (where as demountables break in two) There were two types, the MK 1 having double seat tubes and the MK2 the more common with single tube. I seems unclear if they were ever thrown out of an aircraft but I have seen photo's of landing craft stacked full on the D day landings. After the war many were sold off in cheep lots and then civilianised, losing many of their original parts, mainly the cranks with the bars instead of regular pedals. Some were also made in to sort of lightwieghts fitted with various post war accessories. They were quite a reasonable and affordable ride On the point of "paratroopers bikes" BSA made folders for the first world war yet I guess they didn't have airbourne troops then.
Cheers for now Pete Paine Watford