The actual meaning of this incision is unknown; it could be the Tau/Tav/Taw letter of the Paleo-Hebrew alphabet, a figurative representation of an "X", or or some other schematic figure.

Measurements:

• Seal diameter:  7/16" (11 mm)
• Maximum jar-fragment diagonal:  1 11/16" (43 mm)

One side of the loop appears to have been sliced off lengthwise (since it was washed clean by the antiquities dealer or whoever found it, there is no patina or encrustation to help determine when this happened).

Note:  The 2 cross-bars of the seal impression are raised, though the shadowing in these photos may cause your brain to imagine that the cross-bars are recessed.