There were not a lot of bald Greco-Roman divinities;
Silenos is a major exception -- see
http://www.museum.upenn.edu/Greek_World/pottery_big-36.html and
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On the other hand, your fellow looks more distinguished than usual for
Silenos; perhaps he is
Socrates, who in some ways looked much like
Silenos (Alcibiades' comment in Plato's Symposium) but with ample inner merits to make up for it. It could also be
Homer or some other cultural icon, a sage or a poet; Aeschylos was so famously bald that a bird was supposed to have killed him by trying to
crack turtles on
his forehead. As for your
brooch, it was probably a high-brow status-symbol of sorts, or at least middle-brow; compare modern-day half-busts of Beethoven.