Mercury Switch

I bought this with a 1950's car radio and wondered what it was. A completely sealed cylinder with something moving (not rattling, but moving like it is supposed to) inside. Took it to my old-school auto electrician who was very surprised to see it. He works on old cars a lot but it had been years since he'd seen one. In his opinion it was a mercury switch. Fitted in an out of the way place in the car, it is connected to the horn and turned on when you park. If anyone tries towing your car away, the incline will trip the mercury switch and sound the horn. Small price for an unusual and potentially useful piece of history.
I can now confirm that this is definitely a mercury switch after some noggin-scratching earlier. Thanks to the bloke who called up to confirm that this is what it is.