Doesn't this assume all 1858 reproductions are all dimensionally the same? Cylinder diameter, length, frame window dimensions, base pin diameter, identical hands and ratchets, etc? Ubertis, Piettas, etc? 1970s, 1980s, 1990s, etc? How would one know ahead of time which manufacturer and from which decade would be compatible with any given "drop-in" conversion unit? I've been curious about this myself.