I now own a PRC-64 thanks to Paul, W0RW.  I received this great little radio around February or March of 2015. My PRC-64 (Serial  No. 4) came from the The Australian Army Training Team Vietnam (AATTV) that was a specialist unit of the Australian Army that operated during the Vietnam War.

Checked out my PRC-64:
Channel 1: 1.985mhz Tx: 5w, RX:.5uv
Channel 2: 3.560mhz TX: 5w, RX: .5uv
Channel 3: 5.332mhz TX: 5w, RX:.5uv
Channel 4: 7.030mhz,TX: 5w, RX: .5uv

May, 1965 Army Final Report Summary

The purpose of this evaluation was to. assess the suitability of the AN/PRC-64 radio set for use in the high atmospheric noise level existing in the Republic of Vietnam (RVN) and for use by US SpeciaL Forces patrols in the counterinsurgency environment of the RVN.
Two geographically different areas were chosen for the evaluation. Phase I was conducted in the Mekong Delta and phase II in the Central Highlands. The radios were used by US Special Forces CAC detachments during patrol operations and at forward operational base (FOB) stations in lieu of radio set AN/GRC-109. Data were collected from personal observations, discussions with Special Forces advisors and signal officers, and operators' logs.
The principal result of this evaluation was the determination that the AN/FRC-64 radio set provides reliable continuous wave (CW) communication at distances between 60 and 500 kilometers. It provides reliable voice communication only at distances between 0 and 20 kilometers and then only in the Mekong Delta. The whisper capability of the radio set is reliable and just as effective as a normal voice transmission. User reaction to the AN/PRC-64 was excellent and they recommend its adoption for counterinsurgency operations.
Although the AN/PRC-64 does net provide reliable voice communication at all distances and locations, it was concluded that its performance on CW and weight, size, and ease of operation make it suitable for use by US Special Forces' teams in the RVN as a replacement for the AN/GRC--109.
Australian Role in Vietnam
In mid-1965, Australia's involvement in the war increased as the government committed a full infantry battalion, the 1st Battalion, Royal Australian Regiment. Early the following year, this was expanded as the 1st Australian Task Force (1 ATF) was established, operating in its own area of operations in Phuoc Tuy province. But despite the concentration of Australian forces, the AATTV members remained dispersed, often serving with only one other advisor, either Australian or American. Thus, due to its small size and widespread area of operations, it was rare for the entire AATTV to be in the same place at the same time; this usually occurred only on ANZAC Day – the only other occasion the whole unit paraded together was when it received the Meritorious Unit Commendation from the Commander of the US Forces in Vietnam on 30 September 1970.

That year, as the Australians and Americans prepared to withdraw, a process of "Vietnamization" began, and the AATTV established a jungle training centre in Phuoc Tuy province. In November 1970, the unit's strength peaked at 227, at which time the team was expanded with an intake of corporals. In 1971, the 1 ATF combat units were withdrawn and the AATTV's role reverted to their original role, of training only. As the final 1 ATF units left the country in early 1972 the AATTV, having been reduced to around 70 personnel, remained in Phuoc Tuy to provide training and advisory assistance to the ARVN and to training Cambodian soldiers of Force Armée Nationale Khmère (FANK). The last Australians left Vietnam in mid-December 1972 – the AATTV left on 18 December – following the election of the Whitlam Labor government. The AATTV had the longest tour of duty of any Australian unit in Vietnam, serving a total of ten years, four months and sixteen days. The unit also had the distinction of being the first Australian unit committed to Vietnam and the last to be withdrawn. It was subsequently disbanded in Australia on 16 February 1973. Over the course of its service, a total of 1,009 men served with the unit, consisting of 998 Australians and 11 New Zealanders.
US ARMY Tech Rpt 1967
US ARMY Final Rpt 1965
Delco 5300 vs PRC-64
AN/PRC-64 Tech Info
If you work me using the PRC-64, I'll send you out this QSL card!
WA3WSJ operating my PRC-64 atop Manocacy Hill, PA