I'm sure others will chime in with great info and links, but I'd recommend doing some searches here using the search tool, with some attention being paid to many of the informative threads started by Ted Z., who sadly passed away recently. Pat R has also done some excellent research on the set and started multiple threads (just to name a couple members, but many experts add info to those topics).
This is also a good start when learning about the set and ad backs, Scot Reader's "Inside T206", which has been saved in pdf format and kindly provided on the oldcardboard website:
https://www.oldcardboard.com/t/t206/...al-edition.pdf
also this:
https://www.t206resource.com/
and this:
https://www.t206resource.com/Article...20Century.html