The Duo Sonic (has two single coils) and Musicmaster (one single coil pickup) guitars always had flatwork (Vulcanized Fibre) similar to the Stratocaster. The magnets in the early Duo Sonics and Musicmaster are .625″ long and the early diameter was .197″ until the late 50s, then they were a .187″ diameter. The early ones that I have rewound usually had the sand casting marks on them. This made the magnet look rough and pitted.
The two pickup Duo Sonic guitar was the first that I recall that had a stock (RW/RP) Reverse Wind/Reverse Polarity. In other words, the (Bridge) pickup was wound Top Going with a North Polarity and the (Neck) pickup was wound Top Coming with a South Polarity. This was done to reduce unwanted hum when the two pickups were used together. The pickups used together were actually humbucking. The 50s and early 60s pickups used the same 42 AWG Formvar insulation as the Stratocasters. Later pickups used 42 Plain Enamel for the insulation.
The Duo Sonics and Musicmasters had solid top covers, unlike the Stratocaster covers that have molded round holes so the magnets can protrude through the cover. The magnets on the Duo sonic and Musicmasters pickups are flush with the top of the bobbins flatwork. They are wound quite full like the early Stratocasters and have a great sound. They are not very loud but have great fidelity. They seem a little weak usually because of the solid cover (the magnets are not as close to the strings) and the magnets all being .625″ long. The magnets slightly extend out the bottom of the flatwork.
For many years I have talked to Eric Johnson about these pickups and they’re one of his favorites. Note: Unlike the Stratocaster, the neck pickup on the Duo Sonics are slanted like the bridge pickup.
The Musicmaster guitar has one slanted neck pickup and the more modern Fender Mustang guitar had two slanted pickups.