I was in a friend's car back in 76 which was overtaken by an MGB GT, once passed the driver pulled a bit of string which bought up a sign saying "Police Stop
Cripes ..... was it 1976

Saw a BRH unmarked MGBGT on the Bucks. M1 slip road with 2 bibs inside and it was not too long before he came past me with his bonnet stuck up the tail of a Volvo doing warp speed 9.
A couple of miles on he had pulled him over, strings and box sign in back lit up. The other driver was stamping with rage.
There use to be a crack about the bad guys having a sign which replied 'Bollux - Bandits' !
Regards
Bryan C.