Parker 51 “Red Band”

I have not handled one myself, so I am not totally clear on how the button moves the paddle inside. It appears that the button rests on a compression spring. A bend in the pressure bar arm is pushed through a narrowing causing lateral movement of the arm when the button is pressed and allowing it to move laterally back to resting position when the button is released. Regardless, it is an end activated mechanism for lateral sac compression.