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Field coils are typically on speakers. Permanent magnet technology wasn't mature enough until around WWII for permanent magnet speaker magnets to work, so instead electromagnets (field coils) were used. It wasn't until the early 50s that the last makes stopped making field coil speakers.
In the post war era supply shortages made it so some makers used both types depending on what they could get any given week. Case in point I've got a early post war Zenith AA5 table radio/phono that I used to have 3 of. They stamped the chassis for octal tubes, but used the newer style miniature tubes on some sets and the mix of which were octal and mineature varied depending on supply.
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