From Tom Dixon's book The Chesapeake & Ohio Railway at Mid-Century, "C&O first began using the logo and slogan 'C&O for Progress' in early 1948. As far as we can learn, the first cars to carry the logo were the PS-1 Box Cars delivered by Pullman Standard in January 1948." Tom goes on to say that all freight cars bore the new logo, so any new equipment or equipment repainted after January 1948 would have had the For Progress logo. However, given the amount of equipment on the railroad and the timeframes for repainting freight cars, I'd imagine there were cars that made it into the Chessie era without the For Progress logo.

Tom Patterson