[olddog]

Top picture pistons look to be significantly deeper dished, compared to the bottom picture.

The diameter of the dish in the top picture looks to be a larger diameter dish, as well.

If my observations from the pictures is correct, the top piston is much lower compression. Maybe as low as 8.5:1, which the factory likely used in the late 70's.

If you have both pistons available, fill them with children's putty, and compare to prove it. If you could set them on a level surface the correct way is to fill them with water and then suck the water into syringe to measure the CC.