From the moment I first saw this photograph, I knew I was on the right path. The rifle on the far left in the stack has an A5 scope in Marine Mounts (Mann-Niedner bases). Check out the big, knurled knobs, and one can even see the tip of the clicker-pointer. I believe the Marine on the left is the sniper, due to his gear and position. His uniform looks dark in color, which I believe is due to newly issued uniforms (British French, AEF??) versus the others' sun faded duds. I remember we could spot a newbie in boot the same way. The snipers are probably fresh out of an English sniper school, which is why they have their Brodies. The date of the picture would be late 1917 as all the Marines are wearing Marine uniforms, canvas leggings, and their campaign hats. After January, 1917, the canvas leggings and campaign hats would have been gone. That would make this rifle one of the original Niedner rifles, as the WRA rifles had yet to be delivered. This isn't new ground, as I believe everyone concedes the Niedner rifles went to the 4th Brigade. As JB once pointed out, I think the Marine on the right has pilfered the Brodie from the sniper, as you can see his campaign hat just to the side of his right leg.
Jim