Returning to the valve era and in particular UHF valves, these advertisements give a picture as to what the American valve makers were doing in the first half of that decade.
Firstly, a couple from GE:
Clearly, it was pushing the use of the 6AJ4 as a UHF RF amplifier.
And it was also promoting the use of its UHF valves in combined VHF-UHF tuners. That a pair of 6AJ4 are shown might have indicated that at VHF, the two were used in a cascode circuit.
Next, one from Sylvania:
It was also suggesting combined VHF-UHF tuners. In this case the 6AN4 was used as a grounded grid RF amplifier at both VHF and UHF, and as a grounded grid mixer on both bands. A grounded cathode triode mixer would preferably have neutralization for the VHF low band lower channels when used with a “40 MHz” IF. The latter was more-or-less mandatory at UHF, as the FCC UHF channel allocations had assumed its use. Such triodes (usually half a 6J6 or 12AT7) were used as VHF mixers with the high IF, but pentodes were much preferred because they dispensed with the need for neutralization. But a grounded grid triode mixer would presumably have more protection against regeneration.
And finally, an RCA advertisement featuring the 6BC4:
All quite interesting, but as previously said, only the 6AF4 had any material use in domestic TV receivers.
Cheers,