John,  The stack "compensator" springs are called equalizer springs.
They came in all different lengths, since they attached to different
places in different model pianos.

They are not needed, musically, and the length differences is un-
important to any given spring as long as the distance between coils
is the same. That's what determines tension, anyway.

Their real purpose was to act as reservoirs for the low intensity,
if ever the regulator valve overshot and momentarily touched the valve
tube.  Since it never happens, they became "neat to watch."

Craig Brougher