This is a Leica Standard Model G, serial number 168971, made in 1935 by Ernst Leitz, Wetzlar, Germany. By 1935, most models were chrome, but this is still in black, which was the older style. It had an accessory rangefinder (FOKOS), that swivelled out of the way for adjusting the shutter. Unlike the Leica I, the f/3.5 5cm collapsable Elmar was interchangeable.
