OK, with distributor under #3 cylinder and mark "A12" it is either an imported used non-US 1974-up A12, or it might be an A12A. But b310gx says the engine he saw was definitely marked A12A. So assume it's an non-US A12.

In either case you will need to modify the motor mounts to put this engine in your Datsun 1200.

I wouldn't worry whether it's A12 or A12A about this unless you are talking about racing or boring the cylinders.