HI i think that the plunger that is missing from the cyl stop has broken .Not allowing the trigger to pull down the cyl stop.thus locking up the gun -cyl can't turn -trigger can't be pulled You will have to remove parts to be able to remove the trigger , On the back side of the cyl lock you will find a small disk with a hole in the center ,this holds the plunger & spring in place.The plunger can break the tip off ,this is what the trigger extreme front hooks on to pull the cylinder lock out of engagement with the cylinder.Then as the trigger returns it cams the plunger into the cylinder lock and the hammer returns to reset. The plunger is very small and a smith that is familiar with the model could make one .It is a trial and error to get it right ,i have made one for my gun. HOPE YOU FIND the broken pieces
this is how my 1899 38 caliber S/N 20975 lock work operates

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