Okay. Back Collecting. Now you opened the door. There are a number of advertising backs, often with different series and factory numbers. Piedmont, the most common, has series 15, 350, 350-460, and factories 25, 30, and 42. Sweet Caporal is similar, and also offers overstrikes on the back for factories 42 and 649. Why overstrike? Tobacco factory data was required to comply with federal ciggy sales tax. So, the printer would adapt existing cards by striking the original factory and comply with law, thus an overstrike. I personally think these cards, which will cost you very little more than a more common back, are cool as ****.
A common card with a much scarcer back, like Uzit or Lennox, will take a $20 card into the realm of a four figure cost to you. If you can find one. T206.org lists 41 back combinations for the 524 cards in the set. And, yeah, a master set, of which there is a working functional apreadsheet you can use, does contain several thousand potential combinations. I have not heard of anybody coming close to such an accomplishment. This set is truly a monster. Put that into this checklist. Heh, good luck. You could do it with another column and a drop down box. Kinda.
Some collectors key in on a certain player and collect each back combo for that pose. Others target the 41 backs, and a few tortured souls collect the main set, then collect an entire set of a particular back, such as the 300+ cards with a Polar Bear back. To each their own.