Look at this NFPA Archives NFPA link and see the small box where previous information related to earlier editions of the NEC can be found. In some cases the old TCR, and TCD and other publications are full of some of the reasons for code changes.

I don't think there are any of the old (Red Book) Preprints available though.
NFPA Archives I liked reading the 1940 material.

Hey John S., when was your dad involved in the NFPA NEC process? I remember that he was on a Code panel.
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Joe Tedesco, NEC Consultant