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IIRC, all the flybacks I worked on at Motorola in the late 60s had layer-wound donuts (probably bi-filar) with Nomex paper, and potted in a fire-retardant-containing polyester compound, with a silicone outer "tire". The compound was a balancing act, since more fire retardant was better for flame resistance, but increased the lossiness of the stray capacitance, and therefore changed the tuning and caused more heating in the coil.
At Zenith, they developed a combination of silicone oils that duplicated the electrical characteristics of the potting compound, and therefore there could be a quick turn-around of trial designs, since they didn't have to be sent through the potting process; and furthermore, windings could be changed as desired without throwing away a bunch of cut-and-try prototypes. The engineer's bench looked like a horror movie, except that a flyback would be in the big jar instead of a human brain.
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