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Type B-4 pneumatic life vest. After the outbreak of WWII it was anticipated that supply of natural rubber from Southeast Asia may be compromised. Furthermore, the demand for live vests increased such that adequate production could not be guaranteed by the manufacturers of the Type B-3. Thus, by 1942 the Equipment Laboratory extensively tested the Navy's AN-V-18 pneumatic vest, which was duly accepted by the USAAF, and standardized as the Type B-4 on May 6, 1942. Its construction of superimposed pneumatic compartments made from rubber-coated yellow fabric significantly reduced the need for natural rubber. An automatic CO2 inflation system for both air compartments was activated by the wearer by pulling down the discharge lever cords. It was supported by a separate mouth inflation tube. During initial production an additional strap from the collar to the rear of the belt was added in order to prevent the wearer's head from slipping out of the collar.

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