In public server list first column is named V, and it contains one of following letters: M, N, O, P. What meaning this has?

What version the dedicated server is running.

For example, if a server is "M" then the server runs a 0.6M dedicated server.

...and the variety of versions is because few last versions are fully compatible,meaning that lazier hosters just don't bother to update. You'll see that change when an uncompatible patch comes out - then everyone will be same again.

Quote from Eclipsed :

...and the variety of versions is because few last versions are fully compatible,meaning that lazier hosters just don't bother to update. You'll see that change when an uncompatible patch comes out - then everyone will be same again.

And some people don't bother updating from a test patch version!