Use of the body number in place of the serial number is a common MISTAKE in the world of Studebaker Avantis....It happens, of course, because the actual serial number plate is buried down on the frame rail, and folks that are unaware of that assume the 'easy to find' body tag is the car's actual serial number.
As your State has accepted this 'goof', you're under no pressure to re-title your Avanti any time soon UNLESS you intend to re-sell it....In that event, the perspective buyer will probably realize what happened...and may not want to purchase until the matter is corrected at the registry.
PS...The non-original body tag 'looks' official and is not a problem.