Hopefully this isn't too late for the thread--I've been away a few days.

We've always called these ceramic knobs "Bennington" or "Rockingham" knobs.
They seem to be pretty common in the southeast and are generally readily
available in antique stores at modest prices.  The Knopf Collectors' Guides
to American Antiques, Pottery & Porcelain  dates them as ca. 1845-1900 with
the locality from MA to OH, so. to MD (but we find them often in FL).

                                                        Lucy Wayne