I just got this lens yesterday and am intrigued by the mount . I wonder if this gives any clue regarding what camera it was originally used with. The Lens is a " Dopp Anastigmat serie IB Celor F= 210mm 1 : 5,0 Pat C P Goerz Berlin No 230258" which is a Dialyt construction. It is in a focusing mount so it most likely would have been mounted on a camera with a focal plane shutter.This borne out by the attributes of the Celor which was marketed on its ability to retain edge definition at wide apertures. The lens board is 130mm wide by 85mm high and the flange is 2mm approx thick.See Picture Because the lens focal length is in millimetres I believe this is probably pre 1914. Apertures run from f5 wide open, through f5.6, f6.3, f8 and thence to f 64. It came with its own generous solid metal hood.