Two large Chinese characters Ban Liang ("Half an ounce"), without an outer rim / Flat, no rims. 24mm, 2.57 grams. Gratzer/Fishman "The Early Round Coins of China" #13.68; BM Chinese coins (Poole) #256ff; Hartill #7.16ff (this particular type not listed). 

A Chinese ounce (liang) weighs about 16 grams. The earliest Qin dynasty "Ban Liangs" weighed half that much - 8 grams, but the Western Han pieces, like this one, weighed much less. 

This coin is unconditionally guaranteed to be authentic