It looks to have a shutter, possibly a Dalmeyer fitted internally behind the lens. Dalmeyer made Packard shutters under licence in the UK, they are in my 1940 Kodak catalogue.
Ross subcontracted some AM lens manufacture to other companies, they had taken over control of the Carl Zeiss (London) optical works at the start of WWI. The first AM lenses date back to then, I have a WWI Ross 151mm f16 AM unmarked"Protar" which covers 10x8.
Ian


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