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I have a 21FBP22 branded GE on the base and a 1962 date code. The EIA code is rubbed off, but the "font" of the date code looks more like something RCA used instead of GE, so I think the CRT is a rebranded RCA. I don't think Zenith was making color CRTs around that time, and my 27KC20Q has a 1962 RCA tube in it. Sylvania did make 21AXP22 tubes in the 1950s, and of course CBS/Hytron did as well, being they had quite a few patents in that arena. The vast majority were RCA, at least until the mid 1960s or so.
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Erich Loepke
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