Saxman,
"Thinning of the chamber top" by itself is not a reliable indicator of a ground chamber date.
It is true that early Lugers have a circular chamber profile, concentric with the barrel. At some point in Luger production this was changed, the chamber profile became 'flattened' compared to the barrel, giving the appearance one would expect if the chamber were ground.
I ran into an article in AutoMag in which someone did a survey to determine when the change occurred, regrettably I cannot track down the reference at the moment. A quick examination of a couple of my own Lugers shows that the chamber profile change occurred as early as the 06 model.
--Dwight
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