I think, the difference between the NS12S3 and the A1 version was the head button. I am NOT sure of that. I am almost sure that none of the NS engines had microcast pistons. The NSR and later engines used the microcast pistons.
The Mugen X12 is a CNC piston engine.
For those that don't understand the differences:
CNC is a machining process. In this case the piston is machined from a block of material.
Microcasting is a process where the piston is cast from molten material.
This is simplified but two different processes.