A few weeks ago I asked in this forum if anyone had ever heard of a Zeiss Triotar 210mm f3.5 lens built for 35mm Exakta cameras. None of you had (nor have any of the people who run the various Exakta websites). I e-mailed the nice historians at the Zeiss archive, who (within about 24 hours) replied that the glass was produced around 1925-1926. It is item #1588 in the virtual museum and looks to be part of a 3.5 series made for large format cameras, of which 21cm is the upper limit. That leaves the lens barrel, which seems to have been custom-made outside of Zeiss, but I can't find any branding inside or out. I have made a list of the things that I do know about the lens, and would greatly appreciate any help with the questions that follow: 1. The front glass does not rotate when the lens is focused. Does anyone have any pre-wwii lenses that have this, versus front glass that rotates? All of my pre-war glass rotates, but all I have are relatively basic Leica rangefinder lenses. Do any of you have pre-war Exakta-mount lenses whose front elements don't rotate? 2. The aperture scale is marked 3.5, 4.5, 6.3, 9, 12, 18, 25, 36. Does anyone have an idea of a date at which the standard changed to the more modern 3.5, 4, 5.6, etc.? Usually this seems to be an indication of pre versus post-war in lenses like 50mm Elmars (and their Soviet clones), but I'm not sure about other companies. I know that I have a Kodak 70mm lens from the 1930's that follows the latter standard. 3. The tripod socket built into the lens is 3/8" - by what point had most companies switched to the 1/4" standard? 4. The focus scale is in meters (marked 2.5 to infinity). I guess this points to European origin and might rule out Burke and James...? 5. I can't tell what kind of metal the barrel is made from, but it feels more like aluminum than brass (brass just has that warmer feel and heft to it; I can't quite explain it). When did aluminum come into common use in camera (and other) stuff? 6. The lens mount is an interior mount for Exakta, not the external mount often used for longer/heavier lenses that JDM mentioned in my prior post. Thanks for any ideas anyone has.