AN/PRC 77 Radio Set is a manpack, portable VHF FM combat-net radio transceiver manufactured by "Associated Industries" and used to provide short-range, two-way radiotelephone voice communication. It can provide secure voice (X-mode) transmission with the TSEC/KY-57. The radio consists of the RT-841 transceiver and minor components. In the Joint Electronics Type Designation System (JETDS), AN/PRC translates to "Army/Navy, Portable Radio, Communication."
The AN/PRC 77 entered service during the Vietnam War (circa 1968) as an upgrade to the earlier AN/PRC 25. Today it has largely been replaced by SINCGARS radios, but the AN/PRC 77 is still capable of inter-operating with most VHF radios used by U.S. and Allied ground forces. It is not compatible with the SINCGARS frequency hopping mode. It can be used with the KY-57 voice encryption device for secure communications.
The PRC-77 differs from its predecessor, the PRC-25, mainly in that its final power amplifier stage is solid state and not vacuum tubes.
SOG Recon teams would carry 1 radio with spare batteries spread amongst the team in waterproof bags.
I am still looking for an AN/PRC 25 battery box (doesn't have to be working but 60's dated would be best), so if you have one for sale please get in touch. firstname.lastname@example.org