Appears to have been a high-end classical label. The insert in SR 7301 says:

AMBIPHON RECORDS is a company devoted to the highest standards of excellence in sound reproduction. Using newly developed, wide dynamic range recording equipment and new techniques, Ambiphon has produced a series of tape recordings entitled “Master Duplicates”. These recordings are third generation copies from the master tape and are duplicated in real time on a one-to-one basis. Duplication of reel-to-reel tapes poses many of the same problems confronted in the recording of a master tape, but each of these problems is magnified by the mass duplication processes currently employed by tape duplicators today. The custom-designed duplication equipment used by Ambiphon makes quadraphonic and stereo consumer tape products with a quality far surpassing those of most recording studio’s [sic] master tapes. These results, or “master duplicates”, are achieved by using slower duplication speeds, master quality recording tape as duplication stock, and new recording technology. In addition, Ambiphon does nothing to compress or adjust the artistic dynamic range — no limiters, no gain-riding, no adulteration of the sound is permitted. The artist alone estbalishes and controls the dynamics of the performance.

The tapes include reference tones.