Two to three hundred years ago most Jews in Germany and Holland wore a specific type of hat called the barrette (rabbis wore a different hat). Barrettes were round and flat and made of felt or wool. It was originally a popular hat worn by scholars all over Europe in the fifteenth and sixteenth century. It was first worn daily by Jews but gradually became reserved exclusively for shabbos hence it became known as the schabbes deckel.
The following image (which can be enlarged) is from 1800. Many, many more images from the 18th century (and even into the 19th) show Jews wearing this distinctive hat. (Look at the kallah maidelach to the left, as well as the little boy).
An 18th century example survives (in the Bayerisches Nationalmuseum, Munich):
(images are from here)
This is a portrait of R. Seckel Lebi Wormser, the so-called Ba'al Shem of Michelstadt (1768-1847), wearing a version of the hat. Given his evident advanced age in the portrait, we see it being worn still quite late: