Paul Barth came to Magna in the late fifties as an accomplished guitar designer and engineer. He one of the early collaborators and inventors of the first electric guitars and he held early patents for magnetic guitar pickups. In the late fifties, he designed several models of guitars for Magnatone that were sold between 1958 and 1961, and again between 1964 and 1966. The latter period being in collaboration with Tony Price and Larry Ludwick with the release of the Starstream series of guitars.
Paul was born in 1908 in San Mateo. His parents, Martin Barth and Irma Dopyera, were immigrants who came from the Austro-Hungary region of east Europe. On his mother's side, the Dopyera's were instrument makers. Paul's uncle John Dopyera had an instrument shop in Los Angeles where steel resonator guitars were invented and the Dobro and National instrument companies were born. Dopyera's business was a family affair, his brothers worked there, as did his nephew Paul and Paul's father Martin. They were responsible for inventing several of the first electro-magnetic pickups, include a few patented pickup inventions that Paul had himself.
Barth was an initial partner in Ro-Pat-In Corp., which later became Electro String Inst. Corp. Electro String's primary instrument brand was Rickenbacker. Barth was a designer, and shop manager for Electro String until about 1956. Besides being a electro-magnetic pickup inventor and guru, Barth had a great aesthetic style and was the chief designer for the Rickenbacker Combo guitar designs of the mid fifties.
Electro String was sold to a new owner in 1953. Barth left soon after to start his own guitar company, Barth Guitars. He designed a very handsome guitar with some of the same aesthetic nuances he had designed into the Rickenbackers. There was a music instrument company in Santa Ana called Natural Music Guild, that Paul partnered with, although it is unclear what Natural did for Barth, they might have been a distributor, or they might have helped with the assembly. While body and necking shaping was done on the factory floor at Electro String, Barth might have contracted this work to a local furniture manufacturer for these guitars 
In 1958, Magnatone hired Barth to design their next series of guitars. The first guitar designed for Magna was the Mark VI guitar and bass. It was somewhat similar to The Mark III, which Paul Bigsby had designed for Magnatone a few years before. There were some distinct Barth touches, like the double-cut-away body, pickup design, and control knob location.
The Mark VI was designed by Paul, but Magna was responsible for manufacturing it at their Inglewood facility. All records point to production getting underway sometime between January and March of 1959. Magna had a terrible time pulling this off, and in fact, they didn't. They had to throw away more guitars than they produced. Very few of these survive today. Magna execs (probably Art Buckles or Roy Chilton) went to Barth for help and a different Barth/Magna collaboration was worked out. The result became the Magnatone Mark VII, VIII, and IX guitars. The guitars were made at Barth's off-site facility in nearby Gardena. They were, in fact, re-branded Barth Guitars. Instead of a water-slide decal with the Barth name, the Magnatone name was affixed to the headstock.
The Artist Series, as Magnatone sales literature would call it, launched sometime in the spring of 1959 and included three double-cut-away models (Mark VII,VIII, and IX), a Mark VI bass, and a high end Mark X Deluxe Stereo guitar. These guitars were priced in the $140-$200 range. 
This new arrangement worked out well and the quality of the delivered instruments was very good. Magnatone still advertised the guitars as late at 1961. Sometime in 1960, some desgin changes were made and the names changed from "Mark VII" and "Mark VIII" to "Model 150" and "Model 200".
Since the initial Barth/Magna relationship in 1958, Magna had moved from Inglewood to Torrance, and was later acquired by Estey Organ (more information). The original execs Barth worked were gone, and the new execs were struggling financially to make Estey and Magnatone work. During this time (1961-1963), the collaborative relationship with Barth was somewhat dormant.
Magnatone called Barth again in 1964 to design the next series of Magnatone guitars and basses. Also brought on board at the same time was a good engineer named Larry Ludwick. For the new guitars, Larry designed all the production tooling and jigs. At Electro String, Paul had done this type of work himself, so he must have thought highly of Ludwick. The two were hired at the same time, so it is quite possible Barth had a previous relationship with Ludwick and brought him as part of the deal. Making room for Barth and Ludwick at Estey took some financial rearranging, and unfortunately some of the talented senior electrical engineers had to be let go to make room. Obviously, Estey placed a high priority on the new guitar line.
The Chief engineer at Estey was Tony Price. Price had just finished the design and launch of the "M-series" Custom. When Barth and Ludwick were hired, they were placed in his department, and the guitars ended up being a collaborative effort between all three. The aesthetics and pickup design was done by Barth. Tony designed some of the mechanics including the locking vibrato mechanism.
Production at the Torrance facility started up sometime in 1964 and ran through early 1966 when Estey moved all engineering and manufacturing operations to Harmony, Pennsylvania. All three on the guitar team were offered positions at the new plant. Price took the offer, but Barth and Ludwick did not. After a nearly a year in the hard-scrabble western Pennsylvania, Price took a job in Mexico City. Estey struggled getting the guitar line up and going at the new facility, so they went back to Barth and made him a better offer. This time he accepted.
Barth worked at the Harmony facility and opened his own small guitar shop in nearby Zelienople. After a year or two, Barth left Estey and returned to the west coast where he opened another guitar shop in Riverside. The shop was on the first floor, and Paul and his wife, Frances, lived on the second floor. Paul Barth died of a heart attack in 1973 at the age of 64.
Between the years of 1965 and 1968 Estey made a few attempts to offer some hollow body guitars in addition to the Starstream solid body guitars. Barth was key in the develop of these guitars. While Estey was still in Torrance, Price, Ludwick, and Barth all designed a new headstock with three pegs on each side of the headstock, functionally providing a straight string pull. Price spent a few months in Europe sourcing a manufacturer and finally settled on Framus Werke in Germany. Once the prototypes arrived in the US, Barth fitted pickups to the guitars before they were exhibited at the 1966 NAMM show.
There was another Magnatone semi-hollow body guitar that looked nearly identical to similar guitars that Barth had designed and produced with his California based Bartell company (see below). It seems that Estey might have considered sourcing Bartell for some of these as some point. These guitars were prototypes, and it's unlikely they were really made in any real numbers. It's unclear what transpired. Following the NAMM show, they had outstanding orders for the guitars, and it is possible that they scrambled together some from Crucianelli in Castelfidardo, Italy to fill the orders.
Paul Barth partnered with Ted Peckels to build Bartell guitars in the mid-sixties. I'm not sure when this occurred, but it seems likely that it was concurrent with his Estey/Magnatone relationship. Dates for the Bartell guitars range from 1964 to 1968. Barth was employed by Estey during these years, but nothing prevented him from partnering with Peckels at the same time. I read an account that the Bartell guitar bodies and necks were supplied to Bartell by an outside outfits. Some of the Bartells look a lot like the Starstreams, and perhaps (?) Estey was the supplier of the ones that bore that strong resemblance. For more information, see Bartell guitars.
: This is not confirmed, later at Bartell in the sixties Barth did, in fact, contract with a furniture manufacturer for the shaping of the guitar bodies and necks.
: These list prices were set for head to head competition with Gibson electrics (the Les Paul Jr was $130, the Les Paul was $250, and the Les Paul Custom was $375).