For the Swedish mastering studio use Mastertone Studio, Stockholm.
Mastertone Recording Studios, Inc., a/k/a Mastertone Studios, N.Y.C., was a New York City recording / mastering studio owned by Sidney Feldman, who also served as President of the New York chapter of the Audio Engineering Society (AES). They offered recording services, as well as lacquer cutting & mastering; Feldman is credited with developing one of the earliest mono-compatible stereo cutting systems in 1960 and installed the first computerized lathe cutting system in the USA in 1972.

Lacquers cut by Mastertone can be identified by a runout stamp consisting of a lower case "m" with a small capital "T" on top (see images). Later lacquers are identified with an "MT" etch. These are commonly known as the "trident" symbol. Runouts will sometimes contain 2 sets of numbers (with a dash/dot in between) such as 20-3 (ID) followed by 10-61 (for 1961).

Mastertone was incorporated on July 27, 1953, and closed sometime in 1978.

President: Albert Rashid.
Treasurer: Sidney Feldman.
Secretary: Gene Sayet.

Engineers known to have worked here: Warren Slaten.