[Penthode]

I found the transition to 4.5MHz spacing was done in March 1939, a month ahead of the 1939 New York World's Fair. It was published in Television volume 3 (1938 to 1941) in the paper " A Vestigial Side-Band Filter for use with a Television Transmitter", pages 251-2. Page 253 Fig. 1 depicts the interim 1939 VSB channel with 3.25MHz Video-Aural spacing and the 1939 to 2009 VSB channel with 4.5MHz Video-Aural spacing.

The added video channel bandwidth allowed by the increase in the video-aural carrier spacing from 3.25MHz to 4.5MHz certainly would have improved the horizontal resolution. It was in the spring of 1941 that the horizontal resolution would be somewhat sacrificed to facilitate the 525 line NTSC standard.