Deer Creek should have the replacement fly and the tumbler if one is needed. The half cock position should prevent the hammer from falling. Is there anything in the lock mortise that might prevent the sear from entering the half cock notch in the tumbler. The Trapper pistol has the double set triggers and should have the fly to prevent the sear from going into the half cock notch from full cock to down. Absence of the fly shouldn't show to prevent the half cock notch unless the fly was lost and the trigger pulled often enough when set. When firing the triggers set, the sear rides along the tumbler, the fly prevents the sear from going into the half cock notch. Repeated firing will damage the half cock notch retention slot, and half cock will no longer hold. Look at the tumbler. Will the half cock notch fully capture the nose of the sear? Are the trigger levers too high and preventing the sear in the lock from entering the half cock notch? My guess is that the tumbler is broken from abuse caused by firing the gun using set triggers while having lost the fly.