In your first picture above, there are clearly 3 notches in that tumbler. I think the first notch, you are calling half cock, is the stop where the hammer rebounds, or it's simply a safety notch to keep the hammers off the the pins when the gun is loaded (vs having to use half cock). I think you are never getting to the full cock notch (none of your pictures show a lock in full cock), and for some bad reason you are able to trigger the hammer from half cock, and not getting the inertia you would from actual full cock. Perhaps the sear won't engage the full cock notch because of wear on the sear or the notch itself, has non-fitting replacement springs, or something, but from your pictures you are not getting to full cock. You may find the locks work perfectly if you get them to actual full cock and release them with primed shells in the chambers. You are about a century too late in finding replacement springs. MidwayUSA won't be carrying any. You could perhaps get some custom made, but that will exceed the value of an old belgian wall hanger by a long shot. Maybe it's just a wall hanger. Good luck. I'm done...