Two Chinese characters Ban Liang ("Half an ounce"), outer rim / Flat, no rims. 24.5mm, 3.02 grams. Gratzer/Fishman "The Early Round Coins of China" #13.51; Hartill #-. SKU T1914-50115

"Renzi" liang describes the shape of the character "liang". 

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

This coin is unconditionally guaranteed to be authentic