You have enough information to determine the exact version of Exchange you are connected to. The ExchangeMailboxServerVersion is returning the build number of the server you are connecting to, and you can compare this number to a list of server versions. For example, I know that the build number you reference is Update Rollup 5 for Exchange Server 2010 SP3.
For a list of the build numbers, product names, and release dates check out this article on TechNet:
Exchange Server Build Numbers and Release Dates
I hope this helps. If this does resolve your question, please mark this post as answered.
Thanks,
--- Bob ---