U.S. Army M1A2 Service Gas Mask
The U.S. M1A2 evolved from the Kops Tissot Monro (K.T.M.) > U.S. Army M1 > U.S. Army M1A1. Like the M1A1, the M1A2 had threaded replaceable lenses, but was only manufactured in one "Universal" size. This mask was manufactured at Edgewood Arsenal, MD in 1937 and declared obsolete in 1944.
Similar masks in my collection: YorkCo Smoke & Ammonia Mask, Davis M-1