mongodb/mongo-csharp-driver — pull request #151 by darkiri (2013-02-15 10:23:10+00:00)
Title: CSHARP-672 Derived classes can't put timestamp in second position for server to automatically fill in the value

There should not be any difference now on how Timestamp fields are filled by server in documents with and without discriminator.
Only if SerializeIdFirst option is on.