Difference between the part number and the model number of the MBP?

I have recently bought a Unibody MBP and am looking to upgrade the RAM myself. While searching for suitable memory module over the Internet, I came to realise that there are apparently two ways to identify the MacBook Pro: one is the part number, the other is the model number.

So apparently the 15" Unibody MBP has a part number of MB470LL/A and a model number of A1286. Is it possible that the part number and the model number do not have a one-to-one correspondence due to hardware revision? If that's the case, which identification is more precise and reliable?