Part Numbers:

12010-H1001 PISTON SET-with pin (standard size) (Reg Gas) ALL (exc. GX-H)
12010-18021 PISTON SET-with pin (standard size) (High Gas) GX-H

Interesting. The GX-H part number looks like a Datsun 1000 part number. Is it the same as the high-compression KB10? KB10s didn't have dual-carbs, did they?

Regarding GX, it uses same rings, rods, crankshaft and flywheel as standard A12. In the block, apparently only the camshaft is different (and for some GX, the pistons are different).