Field curvature is influenced by the sensor cover glass. It is flat, but its optical properties are different from air that would be there in front of film. At the center, the light hits it at a different angle and has a shorter way to travel through the glass than in the corners. I'd like to see more tests on film, those posted by jonmanjiro a little while ago don't show this corner thing so clearly.
Regarding the question of how precise a copy it is, I wonder if the maker could get exactly the same sorts of glass or just very similar ones and then adjusted the lenses accordingly...?