I found some info, and here's some cliffs: Harptone was actually a company called Standel (one of the first solid state amplifier manufacturers), who ordered custom guitars from Sam Koontz. Sam was a guitar designer for Framus and also Martin guitars, and he also built guitars himself. It's not clear if any of the guitars he buiit were sold as Harptones. He was apparently a rather prolific designer who also designed some of the machinery involved in making the guitars for Standel.
Around the mid 60's, Harptone worked with Standel to build guitars. Supposedly there were only around 200 total built.
Oh, and as a side note, George Harrison played both a 6 and 12 string version of Harptone acoustics, and I think he used those on All Things Must Pass. Here he is playing My Sweet Lord with a Harptone.
https://www.youtube.com/watch?v=Zm2H0n78BUE