1936 RCA RR-359B Restoration

[Penthode]

The channel spacing is not 2.25MHz. In 1936, the channels were 6MHz spaced.
The 3.25MHz carrier spacing was used from June 1936 thru 1939 when 4.5MHz carrier spacing was adopted.

The figure 2 drawing you depicted is dubious and the information you provided was correct and misinterpreted because the 2.25MHz is referring to the video response not the carrier spacing. It is clear and documented.

And from a purely engineering perspective ithe 3.25 MHz makes sense as it allows the double sideband video and sound carriers to fit into the 6 MHz channel with an appropriate guard band between audio and video.

This is purely academic because as you said the sets have been extensively modified to work on the later standard.