US XM27
The XM27 was created as a replacement for the American M17. The design of the XM27 is identical to that of the M17 with the exception being that it's constructed of lightweight transparent silicone, instead of the heavier black butyl rubber. The XM27 is however found today as an orange mask, as silicone becomes an amber colour over time. It was seen that although the M17 was a protective mask, it was too heavy to carry around and the M17 provided a poor field of vision. The silicone XM27 was far more flexible and was lighter than the M17, which was a factor which proved beneficial to the troops in Vietnam.
Due to its improved flexibility, the XM27 could be packed into a small water proof bag, instead of the larger canvas carrier issued with the M17. The M17 was worn on the thigh of the wearer, which meant there was a high risk of exposing the mask and carrier to moisture when wading through bodies of water. Due to the masks weight the carrier would often slip down the side of the leg, resulting in filter damage from exposure to water.
The XM27 was developed, along side the XM28, which ultimately proved to be better, in 1966 by Edgewood Arsenal when the 1st Cavalry Division requested a compact and lightweight protective mask. The XM27 was later discarded because of its excess weight, the XM28 was favored as it's lightweight and compact design surpassed that of the XM27. Production of the XM28 began in 1968. Since very few XM27s were produced, finding an example of one is highly unlikely.
US XM27. (taken from [1])