Leica fact 18: This is why Leica serial numbers may be confusing in determining the camera model.
On a regular basis I see ads on eBay where a Leica II is offered as being a Leica 1a. If you study the serial number then you would see that it is a 1a. How did that happen?
If you would send your Leica camera to the Leitz factories for maintenance or repair, in the thirties, you would automatically get some new features.
Already from the introduction of the Leica II in 1932 it was possible to have the earlier models to a camera with interchangeable lenses and a build in rangefinder (Emmermann page 171 Jahrgang 1 March-April 1932).