JF Ptak Science Books Quick Post
I do believe that this is a Lewis Machine Gun, one of the most popularly-used automatic weapons by the BEF (and of American design). It was a gas-operated weapon firing a .30-06 Springfield at 500/600 rpm with an effective range of about a half-mile. The drum magazine (not present here so far as I can see) held 47 and 97 rounds. These soldiers were all business.
The original is available here.