The rear screw is likely a stop for the trigger's mainspring. If it were not there, the mainspring would continuously press down on the back trigger forcing its arm hard up on the sear bar, and prevent the lock from being cocked unless the rear trigger is set. The screw allows the rear trigger to float without tension until it is pulled back. That back screw allows you to tune the triggers so that a very short movement of the back trigger engages the front trigger, setting it, yet the back trigger can be under considerable force from the mainspring, and even with this short throw, has sufficient powder to trip the sear.