Bluenoser: Yes- this may be long-winded, but this lock had a series of issues (indicated in part by that notch) which likey occurred in the following order:
The hammer screw was broken off, so somebody had taken the tumbler out and annealed it so that the shank could be peened in order to retain the hammer. After doing this, the shank of the tumbler actually twisted (being dead-soft) through the stress of falling on the nipple, which must have shortened the stroke of the hammer considerably. Since the hammer was peened on, the tumbler couldn't be removed... So Bubba the gunsmith simply filed into the edge of the lockplate in order to file a new full-cock notch and extend the hammer stroke (pictured). This probably resulted in over-compression of the original spring, which probably caused its breakage.
So- that extra notch you see in the tumbler was the original half-cock notch, thus the original full-cock notch then became the new half-cock ahead of the latest one to be filed by the last gunsmith. I've since re-tapped for a new hammer bolt, squared the tumbler shank up and re- case-hardened it, and patched that ugly notch in the lock plate. The latest notch arrangement actually functions fine as it was intended it to, but a new tumbler will likely be in order down the line.