Rear differential identification codes?

[Stevo F]

I'm planning to replace the noisy rear differential on my 2005 Impreza Outback Sport, automatic transmission. From what I have determined, the final drive ratio for this car should be 4.11.

The local junkyard supposedly has two low mileaged rear diffs from Impreza automatics with 4.11 ratio for $55 each, so I'd like to grab one of those. When I'm at the yard, is there an identification code on the outside of the rear differential I can look at, so I can determine if it the correct one before I take it to my mechanic and pay to have him install it? I've read in a couple of places that there may be a two letter code on it (assuming there is what would be the meanings of the codes)