Based on the UMY-25, which was secretly developed for the CIA in 1962. The UMY-25 was designed to be worn and was used for several years by the CIA, FBI and various foreign intelligence services. It was declassified in 1966 and released in a consumer version (the M-60) when the CIA switched to Nagra recorders (Reuvers 2024). A stereo version, of which this is an example, was released as the M-75B in 1967.