  Chrome-vanadium steel socket.  
This hex socket (used with a handle to tighten or loosen hex nuts) is made of steel with a small percentage of chromium and vanadium to strengthen the alloy.  For some reason nut- and bolt-related tools (socked sets, crescent wrenches, adjustable wrenches, spanners, etc) seem to have "chrome vanadium" and "Cr-V" stamped on them at a much higher rate than other tools, even though similar alloys are no doubt used for many other tools.  It's probably a historical thing. 
Source: Farm & Fleet 
Contributor: Theodore Gray 
Acquired: 8 February, 2009 
Text Updated: 8 February, 2009 
Price: $1 
Size: 1" 
Composition: FeCrV
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