Kommando and naturally all other readers.....
this question of one number on the frame and another on the frame plate.....when I resistered my bike in the German system, as opposed to having it with the us forces registered, the inspector who looked at the bike noticed that there was no number stamped in the frame, anywhere. I don't think the bike had to have a number stamped into the frame for stateside bikes, but German law requires a number in the frame inself and just a plate with a number in it doesn't count for much. After all....if this wasn't required, and you decided to to steal a bike, a quick removal of the old data plate and replacement with a new plate with the number of your choice, would give you a legal, but in actuallity, stolen bike in about 5 minutes worth of work.
So the Germans require a number stamped in the frame and if it doesn't have one because it originated from some other land with other laws and requirements, and to the Germans, the data plate isn't worth diddly...there has to be a solution....and the nice fellow stamped a big long number into my bike frame, on the steering head next to the data plate. I had all the paperwork to prove years worth of ownership, and an American title for the bike too..so he accepted it as not being stolen and gave it a number generated by the German system that does those things...a big, long, ugly, unoriginal number.
My bike lost the "legal" association with the number the factory gave it when they built it...all paperwork now goes by this new number. Whether this explains what is the situation with your bike, I can't say, but it could be a simular story....maybe Italy does the same sort of thing....