Most likely late 1957 when there was a systemwide change in fonts and colors.  That’s when heralds were removed, the Roman font changed to an early version of Gothic/Block, the green/imitation Aluminum scheme on engines was changed to “Tuxedo” and open top cars, including flats, went from black to freight car red/brown (which varied over the years).  Black-painted gons were still running around in the early 70s.

Jim King

http://smokymountainmodelworks.com/