If I remember right the depth of the dish of Gen1 piston is 2mm and of Gen2 is only 1.5mm. So Gen2 has more CR if you want have same squish. I put Gen2 pistons and rod into my B12 with 1mm spacer and two foot gaskets for original squish. I workes the combuston chamber for less CR.
The Gen2 piston is lighter and also more stabil than the Gen1, because the ringland is 0.2mm thicker and the upper compression ring has is thinner in the inside, so you have more meat of aluminium to transport heat into the center of the piston. That´s all some years ago - I hope I remember right.