The U.S. Army M9 Field Protective Gas Mask came after further development of the M5-11-7, which also used a 60mm M11 filter attached directly to the cheek.  This design saw many years of use, and was copied by several other countries.  Production began in 1951, but less than a year later the carrier was updated, and the designation was changed to M9A1.  This example's filter port is on the right cheek to accomodate left handed soldiers, and was made by Firestone in 1951.