Post WW II, possibly post 1950 or so. There was an earlier version (again, I think post 1940) of this herald with more rounded script.
Look for the Woodrow Gorman film of his visit to the WP&Y in the late 1940s / early 1950s for some color footage of the WP&Y. I think the box cars and flats were painted red at that time. By the time I visited in 1972, most of the box cars were gone, but at least one in freight car red was still on line, and the box cars used in place of baggage cars on the mixed trains were painted green, and lettered pretty much as your illustration shows.
Charlie
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