  Brake mount. 
This is some kind of a car part involving brakes.  Sorry, I don't know much about cars, what I do know is that this is magnesium, based on its very light weight. 
Source: eBay seller huntinfrek 
Contributor: Theodore Gray 
Acquired: 28 March, 2009 
Text Updated: 29 March, 2009 
Price: $40 
Size: 8" 
Purity: 95%
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