My guess is the bike is over in Europe temporarily, either a military or a tourist owner. You can fly a bike over for a certain amount of time although I'm not sure how long in the EU. People go ADV riding all the time for a few months in different countries. Maybe the owner has rebuilt this one to the point he doesn't trust buying a used one over there and is just sight seeing?
The military would issue local plates like the one on my BMW above. When I didn't want to have my CB750 shipped (free on orders) with my household goods in a container, I paid for having it crated and flown from San Diego to Frankfurt--which set me back two thousand $. More likely, people have their bike shipped in a container. Two months ago, in Austria, I met a guy from Alaska. That is pretty rare. But when you see an old Honda in superclean condition with absolutely no luggage, that is no tourist. People in Europe like to import these old bikes from the States, and they are permitted to keep the stateside license for up to one year after clearing customs and providing the required insurance. I believe that to be the most likely scenario here.
Saw this guy near the Grossglockner, Austria. That's an awesome plate:
But, then again, you won't see one like mine to often either in the Alps.
