Incidentally, the dissertation covered 75 pieces from the Guthe-Michigan collection, plus another 10 artifacts from the Phil. national museum.
As to the basis of the age of the artifacts, I've just leafed through the paper (it's about 500 pages long). The 10th to 15th century dating can be found in the excerpt below.
I suppose they also used the associated Chinese ceramics excavated together with the metal artifacts. These Chinese ceramics provide a pretty accurate metric, and are used all the time ("Nacho" is an expert on this also).
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