Ammunition for M79 grenade launchers and the later XM148 and XM203 was issued in a 6-pocket cloth bandoleer, this wasn't a satisfactory design for the jungle environment in South East Asia as it snagged on undergrowth and dint allow the comfortable carry of a larger amount of grenades.
A Special Forces sergeant in Vietnam assembled a makeshift vest in early 1965 and this prototype vest was shown to a Natick Laboratories representative in Vietnam. The vest was taken back to the States for evaluation and on the 14th October 1965 6,687 of these vests were sent to Vietnam. 10,195 smaller vests were also produced for the ARVN forces. This vest held 18 rounds with six three-grenade pockets tow on each side at the front and tow on the back. These can often be seen in use by Navy SEAL teams.
The vest was good but after extensive testing by ACTIV in 1967 some key issues were raised such as the hard to access rear pockets and the need to use the plastic "egg carton" carriers in the pockets. In 1968 the improved Grenade Carrier Vest was released for testing with 14,400 in Vietnam by the end of 1968.
This improved vest can be seen here and featured the extra capacity of 24 rounds all carried at the fount, each round was carried in its own pocket. The early vests featured plastic snaps and a different weave of nylon mesh to later vests. The later vest, issued from around 1973, also featured metal snaps and 4 longer pouches on the top row for illumination grenades.