Thank you everyone for the welcome and thank you, Coffinmaker, for pointing me to the open range and L.Pettifogger's good write-up and illustrations on de-bugging Piettas. I'll work on this and report back after I shoot it for the first time.
The pin prick explanation may be a disappointment to you. As I touched on above, when my arbor/extractor was tightened all the way, it turned a few degrees too far so that the hand did not align with the slot/window in the extractor collar. The difference was too slight for a shim. I suspect it was originally manufactured this way and not discovered for 130 years or so, because the set screw kept things tight while it was in use, and dried oil, grime, rust, and maybe some paint kept it tight after it got put away--until I started messing with it, of course.
The pin pricks are from an automatic center punch I used to punch the mating surface of the extractor collar. Each punch depresses the center but raises a ring of metal just a little bit around the dimple. It's similar to the way knurling can very slightly increase diameter. It's not very precise, but the liberal sprinkling of dimples raised metal all over the back of the collar and when I screwed it into the breech, it made for a tight fit when the slot in the collar was aligned with the hand.
It's not exactly precision, but it works pretty well, and I always have other, more complicated options if it manages to work loose again.