My understanding is that the 'sear' is a little slide-out piece of metal activated by a linkage or cable that lets a spring-loaded plunger strike the explosive charge. The Macchi MB326 seat had 7 safety pins that were removed before takeoff and one of them was a 'sear' pin to prevent the main gun from firing. That one and a coupla others were hard to get to so the 'groundies' removed them and handed them to the pilot after strap-in.
This is out of the cobwebs of my brain, so happy to be corrected.