Just found a couple of these on US Ebay - I had no idea Gibson used the Mastertone moniker for anything other than banjos - seemingly they did.
Lovely lapsteel -
They had a line of budget guitars which were branded "Master tone" as well.
I had a similar lap steel. Compared to other Gibson lap steel guitars, not the best.
It seemed outside of banjos, Gibson reserved the Mastertone label to the Brands low level offerings.
