Paul Barth was a pioneering American guitar maker and designer. In the early 1920s he went to work at National with his uncle John Dopyera. Barth then moved to make electric guitars with George Beauchamp at Rickenbacker where he collaborated on the design of the early Rickenbacker frying-pan solid body electric guitar. On leaving Rickenbacker in 1956 he started his own company and sold his own Barth branded guitars. These were labelled - Barth Natural Music Guild, Santa Ana California. He then worked with Magnatone in the early 1960s. Paul Barth died in 1973. Don Underwood and Steve Tidwell took over Barth's shop in Riverside California after his death. Barth branded guitars were also made in Japan in the 1960s - I'm not sure how these are connected to Paul Barth.

Source: The History and Artistry of National Resonator Instruments Paperback. Bob Brozman, 1993. Centerstream Publications.

Source: Finding Fretless - Barth Guitars (23 February 2022)