So what does this symbol mean? It's actually Persian calligraphy, and it originally appeared on an earthenware dish made in the10th Century, found in Eastern Iran. It contains a blessing that reads al-yumn al-baraka al-salama, or, "Good fortune, blessings and well-being." A photo of the original dish appears on page 215 of the book, Treasures of Islam, Tabard Press, NY 1985.
And while we're on the subject of calligraphy, perhaps you were curious about that beautiful arabic lettering on the front of the re-releases of Trances/Drones. The calligraphy, by Abdul Fu'ad, represents a chapter from the Koran, called the Ikhlas. It transliterates and translates as follows:
Qul: Huwa-Llahu Ahad
lam yalid wa lam yulad
wa lam yakun lahu kufuwan Ahad
"Say: He, Allah is One
Allah the Eternal
Nor did he beget any, nor was begotten
And there is no comparing Allah to any."