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Nice score, Nick!
RCA did not hold patent on all possible circuits that could demodulate color. GE had an earlier 15GP22 based set that used their own original chroma demod circuit that did not use an oscillator (and GE kept using that thru the end of the tube era). Look at some of the 60's designs like Zenith's beam gate demodulator or Motorola's (or was it Admiral?) single tube self-oscillating demodulator....The former I would not be surprised if Zenith had designed back in their 1954 prototypes. Magnetic convergence was also a Zenith design (RCA's 15GP22 was electrostatic convergence, and they would have kept making them had others not passed them by). The QAM color signaling method it's self was Philco's idea. The reason most makers went with RCA clones is that it was usually easier/cheaper than to make it your self....At least until profitable volume was selling.
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