Pom Pax,
I don't know about m/bikes, but most cars of the period would have one something like this lashed on the running board, which was a very useful feature of cars (we ran a Vauxhall 12-6 [1938 vintage] as late as 1960, which
still had them).
(This thing will run you back £120 - ouch!)
The whole family could perch on them for picnics and shows and the like.
Danny.