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Thanks Bob. That appears to be the missing link.
So the transition includes from 1934 thru early 1936 343 lines with a 4 MHz channel and 2.25MHz carrier spacing. For the July 1936 343 line first public demonstration, RCA engineers moved to the 6 MHz channel where it was first used with double sideband video and a 3.25MHz carrier spacing.
The question remains when the 4.5MHz carrier spacings were adopted. The RCA paper said the first time it was applied to the transmitter was March 1939 which agrees with another paper I read indicating VSB was first implemented just prior to the NY World's Fair in April 1939.
Logically VSB was probably first tested on a 3.25MHz carrier spacing channel for direct comparison with 3.25MHz carrier spacing full double sideband. VSB 3.25MHz spacing would not entirely fill the 6MHz channel and so this would suggest by March 1939 4.5MHz carrier spacing was introduced.
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