Yeah I don't know what happened but if you search for "stamped T" or something like that, you'll find some info on it. In short, it is a proof stamp that shows it was test fired at the factory.
I believe the 1006 also came with two different trigger guards. Both were rounded, but one was more elongated than the other. Not sure which came first and when/why the change was made.
I thought the same thing when I saw it for purchase. At the time, I think I paid $600 for it which seemed about the market at the time. The prices jumped quite a bit just a few months later. I figured it was a bargain regardless of the price because I simply hadn't seen another 1006 or 10mm S&W with the TEN prefix.
It would be cool to find one in each 10mm model with that prefix, if that's even possible.