A904 Accumulator piston spring.

Years ago I knew a old mopar guy who had more than a dozen of those springs lined up on ledge above his shop door like birds on a wire. To remove it or leave it out at overhaul was the poor mans shift kit for years. If the spring is missing the accumulator doesn't work at all so there isn't a need to block the piston at the top of the cavity. Piston just rests at the bottom.