Electrowinning plate. 
This is almost certainly a plate that came from the electrolytic extraction of chromium from its ore, or from an electrolytic purification process.  Chromium was dissolved in a solution and electricity was used to plate it out onto an electrode.  The only difference between this and electroplating such as is done with car bumpers, etc, is the goal: In this case you don't care if it's pretty or shiny, you just want to plate out all the chromium so you can use it for something else. 
Source: eBay seller akilawolf 
Contributor: Theodore Gray 
Acquired: 5 June, 2003 
Price: $10 
Size: 1.5" 
Purity: 99.9%
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