An experimental design from WW I, known as the Kops Tissot Monro (KTM), was standardized as the Model 1919 and renamed the M-1 Service Gas Mask in 1921. It came in five sizes, with fixed round eye lenses, a long corrugated rubber tube and canister, very similar to subsequent designs.
After more than a decade of successful use, two improved models of the M1 mask were developed in the mid-1930s:
*1934: the M1A1 mask with replaceable lenses and improved exhaust valve, minor modifications to the head-harness straps and the mounting of the eyepieces. Five sizes produced.
*1935: the M1A2 mask with a one-size stockinet-covered rubber face mask with a seam at the chin. The universal size fit about 95% of soldiers; M1A1 largest and smallest sizes continued to be used for those who could not fit the M1A2 mask.
Although rooted in the designs of WW I, the M1 series masks (M1, M1A1, M1A2) served until 1944, late in World War II. By 1937, Edgewood Arsenal was producing over 50,000 M1A2 masks per year, but all three variants were in use until declared obsolete in 1944.
The M1 series of Service Gas Masks originated in World War I but incorporated the then-new molded rubber facepiece, covered by stockinette cloth. The rectangular box filter was connected to the mask by a stockinette covered corrugated tube. The M1 eliminated the nose clip and mouthpiece of the WW I box respirators. The problem of lens fogging was reduced by directing the incoming air over the eyepieces. A flutter valve controlled output from the mask.
The M1VA1 carrying bag was used with a wide right-shoulder strap, carried at waist position on the left side. In use, the mask was removed from the bag while the canister remained in the bag, connected by the hose. A waist strap kept the bag close to the body.
M1A1 masks will be stamped US on the forehead with a size number under the US. The M1A2 mask was stamped US with a "U" or "Universal" under the US. To differentiate the masks in photos, the lowest head strap on the M1A2 is higher on the facepiece, at about the level of the bottom of the eyepieces. The lowest strap of the M1A1 is much lower.
The filter canisters were replaced with improved models over the years that the M1 series was in service. Early canisters were designated M1XA1, and were included in the nomenclature of the mask. For example, M1A2-1XA1-1VA1 to designate the facepiece, canister, and bag components. This nomenclature was dropped after about 1940, replaced by Mxxx designations for each of the components separately.