Mike,
Scot Reader's "Inside T206" is only available, I believe, via download. It is in my opinion, the leading source for information on the set. Also, board member Ted Z has made numerous very informative posts on this board regarding the breakdown of the set. His research has been invaluable to those of us collecting T206 cards.
Regarding the backs, here's my quick explanation:
First, you have each of the different brands - Piedmont, Sweet Caporal, Polar Bear, Tolstoi, etc...
Then among each brand, you have the different "series" - 150, 350, 350-460, and/or 460. Some brands, such as Polar Bear, did not identify each series. Remember, T206 was a three-year issue.....1909-1911; Series 150s were made in 1909, whereas Series 460 came along in late 1910-early 1911.
Then, among each series, you have different "factories" - 6, 17, 25, 30, 42 and 649. Again, not all brands used all the factories, i.e., Hindus came only from Factory 649, and all Polar Bears were made at Factory 6.
The "overprints" are simply cards where the original factory number was obscured, and a new factory number was printed, normally below the original one, on the back of the card.
Steve
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