While all of the things above are valid indicators of a true A12 car you have to remember it is 40 plus years old and a lot can happen to a car in that time.
The key thing is the M engine code in the VIN. Don't confuse it withe the 2 digit of the VIN as any 1969 RR or Bee will have an M there. It is the 5th digit.
As said above check the hidden VIN numbers to match the VIN plate and you are very likely assured it is real. However in 40 years it could have been front clipped, rear clipped or both and those numbers may not be there or if used parts were used it will have the VIN fron that donor car. Not to mention that there were some cars that the hidden numbers were mistakenly missed when but that was very rare.