Mastertone was a Gibson brand used for banjos, acoustic guitars and electric guitars. The brand was first used in 1923 when Gibson introduced a new line of banjos - these used a new design with a hollow metal tone ring which had holes on the inner and outer sides. The tone ring was rested on ball bearings which raised it up off the rim. Gibson also used the Mastertone brand for some acoustic and electric lap steel guitars.

Source: Gibson's Mastertone Banjos. George Gruhn & Walter Carter. Vintage Guitar Magazine. June, 2009.