2deuces - That's interesting that all the cards you opened were the white card stock. Jeff's assumption that Topps used up whatever they had from a previous run could be correct. Not just from a previous baseball card run, but from whatever other non-sport sets they were printing at the time as well.
I have also heard the theory that Topps printed cards in 2 different plants (one closer to east coast and one closer to west coast) to supply either coast easier. That might make sense why your packs were all "white backs". Those might have been what was supplied to the east coast and the "tan backs" on the west coast. Who knows.
No doubt though that they are 2 different card stocks, for whatever reason.
If you look at one of each, under magnification, you will see that the white stock is noticeably thinner. You will also see that the center of the white stock, sandwiched between the top and bottom, is actually dark grey. While the center of the tan stock is tan like what you see on the back.
Now, a couple ??'s come to mind:
1 - Since these are 2 different card stocks, either printed at different times or in different locations, was one printed in greater numbers then the other?
We may never know since Topps, as far as I know, has never disclosed their production numbers. Just looking at random cards on ebay, that show a front and back scan, the "white backs" seem to show up less frequently. But my random searching donn't mean nothing. It could be totally the opposite. And I'll bet most dealers, and probably most collectors, don't care one way or the other.
But here is my dilemma since starting to notice these.
When looking at my fledgling set from the back, there are white backs and tan backs mixed. I'm not sure I like this. The OCD in me is now telling me to either pick one type over the other, or make 2 sets (twice as much $$).
Here's the problem however. I prefer the look of the white backs, but they are thinner and might be more susceptible to damage, making decent copies that much harder to find. But the tan backs seem to be easier to find. What to do, what to do (thinking aloud)?