It's something I've wanted to study more in depth at NARA but haven't got to yet. My gut tells me that the newer aircraft were the first to go to the silver paint and those already in-service did so during major overhauls or not at all (paint can change center of gravity so I suspect the regular maintenance crews didn't repaint aircraft for the most part).
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Tracy White -
Researcher@Large"Let the evidence guide the research. Do not have a preconceived agenda which will only distort the result."
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Barbara Tuchman