The spring and plunger are actually to push the bolt (and thumb piece) forward so the bolt locks out the hammer and action from functioning with the cyl open.
The wide part of the bolt where the screw is located moves forward under the pointy rear of the hammer so it cannot be cocked. That's why with cyl open, if the thumb piece is pushed back, the action will function for diagnosis of action issues by eliminating the cyl from the equation.
This method replaced the 1st action lockout on the yoke shaft which locked out the trigger on the very earliest 32 Hand Ejector Model of 1896 that did not have a thumb piece or cyl bolt.