I remember reading on some lightbulb collector's website (yes there are people who collect old type lightbulbs) who said that the London Tube uses a special type of bayonet fitting for their signal light bulbs (to discourage theft).

Apparently the bulbs have three locking pins on the base instead of the usual two. Not that there's probably any trouble with taking a needle file and obliterating the offending pin. [Linked Image]