Toronto. Did you ever wonder why the original Leica I with a “hockey stick” infinity lock and a non-interchangeable 5cm lens (or any I or II for that matter) is so darn hard to find? The answer lies with the old Leitz factory. In 1930, when Leitz came out with interchangeable lenses, owners of the original Leica Is could return them to the factory and have the lens and lens-mount replaced.
In a similar fashion, Leitz would retrofit Leicas at the factory to add slows speeds and/or built-in rangefinders. Only a careful scrutiny of serial number tables can show if your III, or IIIa, or IIIc, or IIIf began life as a I or II model. All changes were factory made. Leitz sold versions designated as models I or II to allow people to buy less expensive models of the Leica. Accessories were sold to add slow speeds or a rangefinder ability if it was later deemed needed and a factory retrofit was not practical.
Similarly, while post war lenses were usually coated to improve contrast, prewar and war time lenses were often returned to the factory post war and coated. This made the contrast far better but made dating a lens difficult unless a serial number list was in hand (that too was an issue since some lenses were assembled post war from prewar parts).