This interesting discussion will be usefully extended by reference to the definitive study which was published in 1985:
The American Railroad Passenger Car (Johns Hopkins Studies in the History of Technology) by John H. White Jr. (Johns Hopkins University Press, 1985)
Frank Conlon
Thanks for the corrections and additional information.
But were not fhe frames for thfse 1929 office cars the same as for P70s?
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What was told me the when I rode the car on a BERA (Shoreline Mus.) charter.
Note that many of the small windows in the clerestories were of tinted glass, green being the most popular but generally a color to harmonize with the body paint. I seem to recall that some cars assigned to top trains such as the Empire State Express actually used stained glass. Tres ekegante!