[L78Kiwi]

Woops I think you are confused between the 2 cars shown in the photos above. My 67 has an M21 not an M20.

As for the VIN code stamped on the diff, yes its odd. But the original owner explained he had to waited 3 months for the car due to 4.56 gears being a special factory order. So I assume this is why the diff casing was stamped.
I do plan to drive the car and so in order to reduce wear and tear I will change the gears to 3.70....not to mention other practicalities. I have the original 4.56 gears but they need replacing anyway.