Returning to American UHF TV tuners, the 1964 model year does seem to have marked the beginning of the transition from valves to transistors. All TV receivers manufactured after 1964 April were required to have UHF tuners. Electronics World (EW) ran an article about UHF tuners in its 1964 June issue, page 28ff, available at:
http://www.americanradiohistory.com/...aster_Page.htm.
Given as typical example circuits were both valve and a transistor models from Standard Kollsman:
The Standard Kollsman UT transistor UHF TV tuner had been announced earlier in 1964:
Although the oscillator valve in the valve model was not identified in the EW article, it was probably the then-new 2/3/6ZD4. Standard Kollsman was using the 6ZD4 in its UHF converters as advertised earlier in 1964:
RCA had announced the 2/3/6ZD4 during 1963. So even with transistors on the horizon, RCA still thought it worth developing an improved UHF oscillator valve to supersede the erstwhile 6AF4A.
Cheers,