Full disclosure before the story: I never found the origin. The correction I ended up shipping says "this number does not reproduce from the source", which is a weaker claim than "here is the bug that produced it", and I'd rather admit that than pretend the case is closed. The number is 1,803.
Source: [Dev.to](https://dev.to/mahirhir/the-same-wrong-number-was-in-six-files-and-i-still-dont-know-where-it-came-from-4fhg)