I was thinking the same thing -- and I am intrigued that Rabid Shaver has the same card as mine. I'm still wondering if this was a single sheet of very dark. Will be interesting to know if anyone finds a very dark tan chase card.
I did not find any of the dark tan back stickers, but I have a possible explanation as to how this may have happened (pure speculation, of course!)
Since the "tan" backs are actually thin white stock with a process-printed (simulated) tan stock, it is possible that a few sheets were double printed, causing a darker tan print as there were 8 plate passes instead of the usual 4.
One wrench in this theory is why were there not more dark tan-backed stickers found in individual boxes. I don't have a clue as to how these are collated into the packs ~ especially since OS1 & 2 had a pretty predictable distibution pattern after opening a few packs.
Like I mentioned above... pure speculation! Even if it IS a print annomaly/error/fluke, it would be interesting to see if a complete set of dark tan-backed stickers could be assembled!