There were two incarnations of the accumulator, '60-'70 and '71-later. On the pre-'71, line pressure acts on only two surfaces of the accumulator piston but, starting in '71, a new fluid circuit was added that is controlled by the new 1-2 Shift Control Valve.

In all configurations, the outer accumulator spring serves only to cushion the application of the front clutch and has no function on any upshift. On the 1-2 upshift the fluid pressure acting on the piston offers resistance to the piston's movement and provides a cushioning effect.

These pics are of the early piston location in both 1st (top pic) and 2nd (bottom pic); the blue fluid is line pressure.