The battery powered PRR-9 helmet mounted receiver was used in conjunction with the PRT-4 handheld transmitter as a ‘walkie-talkie’ type radio. After successful testing, PRT/4 – PRR/9 sets were first sent to Vietnam in March 1967. However, the sets performed poorly in the field compared to the PRC-25 and were consequently relegated to base security use.11. U.S. Army Uniforms of the Vietnam War by Shelby Stanton (Stackpole Books 1992)