It came off a Mavic front wheel of a 1980/90s Peugeot road bike that I'm renovating.
It's about 2" (50mm) in diameter and has three prongs that clip through the spokes around the rim of the hub.
The grey bit is plastic and the brown bit is magnetic.
Anyone got any ideas?
http://farm3.static.flickr.com/2457/3941864636_12eee1c55a.jpg

