[ChrisW6ATV]

This is a good and interesting question.

You may already know that the TRK-9 and TRK-12 from 1939 also have 1940 versions called TRK-90 and TRK-120. (One main difference I know in the 120 is that its radio chassis does not have the eye tube that is in the TRK-12, but I do not know of specific TV differences if any.) Rider TV manual #1 has service information for the TRK-9, 12, 90, and 120, and if I remember right it does include two separate tuner modifications that change some or all of the frequencies of the five channels those tuners receive. One or both of the mods mention that the channels will now be 2-6 rather than 1-5, and I know there are TRK-12s that do have a decal with the new channel numbers.

I do not remember any mention of circuit modifications for FM sound (they do mention slope detection) or for the horizontal scan rate change. Also, there is definitely no mention of any TRK-120s built with FM sound circuits or 15.75 kHz horizontal sweep as standard. I also do not remember if either or both tuner modifications were from post-war or not.

What seems likely to me is that TV sets were not selling enough for manufacturers to design and release new models after the initial group of 1939 designs, other than the two 1940 RCA ones I mentioned.

It would be good to find information on TV broadcast schedules and TV set sales in the USA between 1939 and the end of 1941. Were they expanding, or stalled/shrinking?