The problem appears to have been identified. (See above.)
Apparently, it was simply a loose screw. Not me, no.
Internally, in the area that holds both the magazine release spring and the trigger bar that drives the hammer. After a simple tightening, it went back to it's same, old, great reliability.
Angus Hobdell's got it in his mitts, right now, so he's giving it a thorough going over. I've asked about how to avoid such things in the future, what precautions to take, etc. Hopefully it'll be wrapped up soon without anything else being found. It would break my heart if some real damage were found.
More soon ...