Hi David, your Manta is 100% sure a real G/TE. On the "fahrzeugbrief" you will find the "schlusselnummer", has got nothing to do with the number on your key 😁, but is an identification code the Germans use. This code is on your Manta 547 and thats the code of a G/TE👍. Your car is made in Antwerp in the 11th month of '77 and exported to Germany. The production of Manta's in Russelsheim stopped somewhere in October '77.
@Steve 1900SR, I noticed this also. Probably the VIN plate was made in West Germany?