Good question!
 
For years I've been seeing similar marks, usually small geometric shapes ...
the one I remember most often seeing is a little "daisy" or asterisk-shaped
impressed mark.  Memory serves that they are more common on pearl or early
whitewares (ca. 1820s or 1830s).  We don't get much earlier here in Michigan.
 But I don't think these type marks are exclusive to the earlier ceramics.
 Always assumed that they were simply in-house marking devices and doubted
that they would have any real use for manufacturer or date attribution.
 
George Miller... Where are you when we need you?
 
Mark C. Branstner
Great Lakes Research Associates, Inc.