I think there used to be a flimsy security chain that was available as an optional extra that would fix somewhere down in the hole and could be drawn out and wrapped around the wheel and locked. When not needed it could be stored within the frame. I've never actually seen one and don't know anybody who has but was told the above by the salesman at Kawasaki City West Brom back in the 80s. By all accounts a good pair of nail clippers could cut through it so surprisingly not many got sold. Hopefully someone else might be able to verify this story or maybe the salesman had been on the tramp juice.