The foundational fixed-lens Leica I model A was being replaced by the new Leica C which accepted interchangeable non-standardized screw-mount lenses, each of which had to be individually matched to a specific body and weren’t rangefinder coupled. How did it happen? Well, the period between 1930-1932 was a an intensely active time of rapid transition at Leitz. There are also a number of early black finished 90mm f/4 Elmars (affectionately known as Fat Elmars), and a much smaller number of exquisitely petite early 35mm f/3.5 Elmars, neither of which have the usual serial numbers engraved on their front lens I.D rings. The most common ones are nickel finished 50mm f/3.5 collapsible Elmars with no tiny numerals on the front of the black aperture setting ring. Leitz Wetzlar was almost fanatically fastidious about engraving serial numbers on its cameras and lenses (even accessories!) and in keeping meticulous records that have proven to be of immense value to historians and collectors it’s kind of astonishing that there are a fair number of vintage Elmar lenses floating around that are utterly devoid of serial numbers in the usual places. Yes, but they weren’t oversights, but rare artifacts of a transitional era

Leica 35mm summilux serial numbers