From July 1936, the RCA television field trial was tested on the FCC authorized experimental frequency band: 44 to 50MHz. In 1938, the FCC authorized the band 39MHz to 44MHz for Armstrongs FM experiments.
From July 1936, RCA television utilized on the 44 to 50 MHz band a video carrier frequency of 46.5 MHz and an aural (sound) carrier frequency of 49.75 MHz. This meant the video audio carrier spacing was 3.25 MHz and the video bandwidth for 343 line interlaced at 60 fields per second was 2.5MHz.
Video Modulation: Amplitude Modulation (AM), double sideband.
Audio Modulation: Amplitude Modulation (AM), double sideband.
343 line television used thru 1936-1937 was pre Kell Factor. The introduction of the Kell Factor in late 1937 early 1938 allowed for the increase of lines to 441. Video transmission was still AM double sideband with the same 3.25MHz video-aural carrier spacing.
In April 1939, about a couple of weeks prior to the the television demonstration, RCA switched to VSB with the video- aural sound spacing I believe at 4.5MHz at that point. There must have been a jump in horizontal resolution which was maintained until 1941 with the NTSC adoption of 525 line and reduced relative horizontal resolution with the 0.6 Kell Factor.
Note that the UK 1934 BBC 405 line standard was designed with pre Kell Factor of unity. I recall reading of CBS engineers visiting the UK in the 1950s viewing BBC 405 pictures and remarking how sharp the images were. (Although that may have been mostly due to the 4.5" image orthicon tubes the British were using at the time).
So the move to VSB was not until April 1939 which means the 1937 set most certainly would have had a 3.25MHz sound spacing and only a 2.5 MHz video bandwidth. If a 525 video was to be applied, the pictures will be quite soft unless the amplifier design is modified. Also because the IF band in this set was from around 8 to 14 MHz, altering the response to achieve a good vestigial sideband response with accurately place Nyquist slope could be a bit of a challenge.
Lastly the horizontal sweep frequency of 343line was 10,290Hz. (Not far from the 405line horizontal sweep rate of 10,125Hz). Moving it to 15,750Hz and ensuring efficiency and linearity may be (or once was) a challenge. My guess is some of the changes were done likely professionally initially by the factory and later hacked for NTSC 525 and FM sound with the hacked new tuner.
I expect you will discover interesting attempts to more or less enable the set to cope with the later NTSC signal. But it would be interesting to see it perform as originally intended on 343 or 441 lines.
The information on the 1930s RCA television development is well preserved in the RCA publication of Television in five volumes. I have attached a link to Volume 1 covering 1933-1936.
https://archive.org/details/televisionvoli1901unse