Problem H License plate, Fgstnr. does not fit

Hello from Dresden ! Have a quite big problem ! Have built me a Moskvich 412 from 1977 ! I presented it yesterday for full acceptance incl. H license plate review ! Result : Everything in order and suitable, except that the coach has a different chassis number ! I was told when buying this would be no problem ! Well ! Nice crap, a pile of work and money beat up ! Well, was today at the registration office and they told me , I needed a proof for this ! by some previous owner ! The car had 2 previous owners in Berlin, but I can’t find them (dead, moved etc.) ! Now my question : What now ?? What can I do now to let the car still be called Oldi ? Would be great , if someone could give me a tip ! MfG Starpeople44