When the United States entered World War One, there was no formal sniper school or training in place. However, the US Army had adopted a sniper optic for the M1903 Springfield rifle. It was the M1908 Warner & Swasey Musket Sight, a prismatic, 6x magnification scope. It was shortly updated to the M1913 configuration, and several thousand of each type were mounted to M1903 rifles, as well as M1909 Benet-Mercie light machine guns.
Theseoptics were not particularly good by modern standards, as they were heavy and suffered from fogging and condensation (they were not sealed designs). However, they were the equipment available to the Army at the time, and were put to use. This particular example is a 1913 pattern scope with a locking ring from the earlier 1908 model.
Perhaps the most interesting elements of the Warner & Swasey are the data plates affixed all over it, providing ballistic data for the .30-06 ball cartridge which was in use during WWI.