If your use of the name came after the federal registration, you just might be in trouble.
The bottom line is that when you register an FBN, they don't really check to see whether or not your FBN might be infringing on any trademark (since that's not the job of the registering agent). Additionally, having an FBN gives you no real right to use the mark which would 'trump' any validly-registered trademark or service mark. If it did, a federal registration of any trademark would be almost entirely pointless, as a business owner could simply register an FBN in some state or county jurisdiction and proceed to use the name, having it 'trump' the federal trademark registration.
Simply put, you're not finding any such authority because none exists.
Assuming your use of the "Harry's Restaurant" name did not begin prior to the FBN registration, your use came after the registration of the FSM, and your use of the name could very well be trademark infringement. If this is the case, and you wished to continue being 'Harry's Restaurant', you could attempt to sue for a cancellation of the mark (although you should expect a counter-suit for trademark infringement) or else simply hope that they don't ever notice your potential infringement (this is not recommended -- liability for trademark infringement can be ruinous to a business, especially if they know of the mark they're infringing.)
On the other hand, if you were using "Harry's Restaurant" in commerce prior to the registration of the FSM, you may continue to have some right to use the name.
In any case, you should be taking a face-to-face consultation with a trademark lawyer, especially if you suspect that the FSM holder has noticed you and is possibly in the process of suing.