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For reasons now forgotten I mailed an inquiry to Con-Cor about the flat car being discussed, sometime between June 1982 and September 1986. Con-Cor was, at that time, partly in Illinois and partly in Arizona.
Someone from Con-Cor called me to answer my question. Discussion ensued and the gentleman told me Con-Cor had made a new underframe for the flat car. He sent me one gratis.
I presume all subsequent deliveries of the Revell/Con-Cor flat car included the revised underframe.