Why is shield called a "head of bronze halberd" in Terra Cotta Army?

The "head of bronze halberd" shown above is a shield. It is shown in "The Qin terracotta army" by Zhang Wenli The backside view plainly shows a vertical handle. It is nothing like the halberd blades of the time. It is like the shields of the time, and is somewhat similar to the medieval European Pavise. Thank you, Charliebear