We find similar marks--especially circles, "daisies", and other odd impressed
marks down here in SC on pre 1850's "pearlware" pretty often--especially
shell edge. I am sure that I have seen one such mark--a "Daisy"--on a plate
with a Clews Cobridge mark, though I can't give you an exact  reference to
it. I tend to agree with Mary B and George M--these are likely potter's marks
used to identify an individual workers' wares. Let us know what you find
out....Carl Steen