Original U.S. Vietnam War Hand Held AN/PRT-4A Radio Transmitter Spanish Military Equipment and is marked on both

and is marked on both the top of the barrel and the lock (faint) by the maker:

From the data plate visible on the inside of this unit

This piece is a real treasure as it is not constructed from the later war aluminum

and is marked on the bottom with U

It is marked with an inspector stamp

Original U.S. Vietnam War Hand Held AN/PRT-4A Radio Transmitter Spanish Military Equipment and is marked on bothOriginal Items: Only One Available. Introduced in 1965, the PRT 4A is a 2 channel crystal controlled handheld FM transmitter, for use with helmet receivers as part of a set. The A version transmits a 150 Hz tone that opens the squelch on receivers so equipped, such as the VRC 12 series, the PRR 9 (XE 9),and PRC 25 77. Power output is 450mW on Channel 1 and 50 250mW on Channel 2. The radio has an empty skeleton detachable bottom BA 399 U battery