The flag of the Seleucid Empire
The flag of the Seleucid Empire is, as can be seen, Seleucid purple with a strange symbol on it, in a different color of purple. The higher part of the symbol is a helm, but what the lower part is, is unknown. However, the middle part of the symbol, just below the helm, could be another helm. Perhaps it isn't 1 symbol, but 2, or perhaps even 3, linked together. We probably will never know.