That's a good question. The earliest use of the shield logo I've encountered is in the March 1946 issue of Railroad Magazine, accompanying a short article by Frank P. Savage, entitled simply "Two-Foot Gage" [sic], that recalls the author's childhood memories of humorous mishaps during the construction of the F&M in the 1880s. (It's amazing to realize that these events of 60 years before were still a matter of living memory in the 1940s!) It was Railroad Magazine's style in this era for non-fiction pieces like this to feature the corporate logo of the railroad being written about, so the editors appear to have believed it to be the real thing -- a mere decade after abandonment. But where they got it from, who knows?