NOTES: The US Army M9 Field Protective Gas Mask is perhaps one of the most successful gas mask designs ever. It remained in use for approximately two decades and it has been copied by at least five countries. The only difference between the M9 and M9A1 are the carriers, the masks themselves are identical. The M9 design, was basically an upgraded version of the US M5-11-7 Assault Service Gas Mask. It used the same 60mm M11 drum type filter canister that was attached to the side of the mask. The M9 was most commonly manufactured with the filter inlet on the left side, however, a right side filer inlet version was also made. This was to accommodate left handed personnel during weapon sighting. Because the M9 was in service for so many years.