Any updates on this johnsock?
hwall is correct about the stepping from the iFixit picture. The SSPEC from the chip package does indeed indicate B3 stepping. As for checking a machine without cracking it open, I've found where to look in IORegistryExplorer.
Here is a grab from my 2010 MacBook Pro. It shows where to look in IORegistryExplorer. The relevant data is in the revision-id field:

NOTE: This will not be exactly the same as the 2011 model since this is from my 2010 one. If you look at this field for the 2011 model, the revision-id will indicate <05 00 00 00> for the B3 stepping. My data source is the same 6 Series Chipset Specification Update that hwall used to look up the SSPEC.
Please post a capture if you are able to show what you have. I've requested this info from the same guy that got me the last capture from IORegistryExplorer. I'll post anything I get as soon as I have it.
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