I remember a piano repairman telling my parents (when I was way too little to understand) that replacement parts had to be within a certain window of serial num

I remember a piano repairman telling my parents (when I was way too little to understand) that replacement parts had to be within a certain window of serial numbers to fit. It wasn't because of deliberate changes in manufacturing, but had to do with wear and drift in the templates (or molds or presses or whatever). He described it as a "living" process, and that it worked because it cascaded in a complementary way; a small deviation in the shape of one key was "absorbed" and compensated for by the next key. It also prioritized consistent tuning (result) over persistent shape (method). Put another way, the manufacturing process had built-in methods for self-correcting systematic error. They just weren't guaranteed to be consistent in the long term. (I'm sure there's got to be a more technical description for this, but it's beyond my knowledge). I know semi-modern (opto-mechanical) survey equipment is manufactured on similar principles of self-correcting deviation (although these also tend to be dependent on strict method of operation for it to work itself out). It wouldn't be a stretch to think a firearm as old as the 1911 was manufactured in a similar fashion as some point. Especially if there was a way to keep the long term deviation centered within spec. But that's just my guess.