An Achaemenid round shield?

Several years ago, Nadeem at Eran ud Turan shared this ceramic horse and rider from the Pergamon Museum, Berlin, and I've been wondering about it ever since.  The rider bears a round shield with a boss — something I have never seen in any other Achaemenid context.  I haven't even seen a domed Achaemenid boss outside of antique dealer websites, which I don't put any stock in prima facie since they are usually lacking evidence for how such objects are dated.  I've sent a message to the museum asking them for further information; in the meantime, we can only speculate.  Domed round shields have been or could be made of wood, leather, rawhide, wicker, and spiraled cane.  If the shape of the board is merely an artistic compromise to prevent the ceramic from being too easily broken, but the real thing was flat, then it could also be made from vertical stickwork.  None of these possibilities preclude a boss made from a different material.