Here is a David V. Diamond
http://profiles.ehs.state.ma.us/Profiles/Pages/PhysicianProfile.aspx?PhysicianID=9206&Print=True
The article must not be correct, or you are correct that he is not registered. I did a Psychiatry Licensing check in Dover NH and there are 32 registered, none are him.
I also checked the NH state registry and could not find him. However there is some name confusion as there are at least two different David Diamond psychiatrists in MA with different middle names.