I Have an interesting observation about that most pedestrian of Leica lenses, the long-lived 4 element 90/4 Elmar. Other than coating, does anyone know of changes in the optical configuration to this lens (prior to release of three element version)? <p> I ask this question because I believe there was. Yesterday I was examining three examples, two postwar coated A36 SM lenses (636,xxx and 874,xxx) and a later M mount lens (1713,xxx). The SM lenses appear optically identical, but the M lens is different. Specifically, when a SM lens is mounted on a Leitz 90mm SM to M adapter, and placed on a table next to the M lens, with both set at infinity focus, lens units screwed on firmly,the M lens is longer. I'm not referring to the lens mount, but rather the front of the convex front element is several millimeters higher in the M. This is hard to measure, but easy to see. The front element in the M is less convex (flatter) and when I look directly into the lenses' front elements my reflection appears upside down in the M, right side up in the SM. <p> Thoughts?