Over in India the lockable petrol tap is still a popular accessory but comes with a key. They're pretty good devices, I've fitted a couple to Indian Enfields.
The CEAL tap dates from the twenties. The concept is simple, the tap has no clear on and off, petrol will only flow in one position of the ten available. To set the tap from new you take off the dial, find where the open position is and then put the dial back on with the number of your choice facing the on position. Of course the security flaw is that the thief could just take the dial apart or try flooding the carb in each of the ten positions. I guess it's a mild deterrent rather than Fort Knox level security.
A nice touch with the CEAL tap is that there is a gravity filter that you can easily take apart to remove sediment from time to time.
I'm all for period accessories, way more interesting than standard catalogue restorations and more evocative of bikes that people actually rode. I bought a pair of these and they will eventually find their way on to a couple of vintage projects lurking in the shadows of the workshop.