Two Chinese characters Ban Liang ("Half an ounce"), outer rim / Flat, no rims. 24.5mm, 2.39 grams. Gratzer/Fishman "The Early Round Coins of China" #13.51; Hartill #-. 

"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. These late Ban Liangs were cast during the reign of Emperor Wudi (140-87 BC), the date of the casting is ca.136-118 BC and they come from the last issue of Western Han dynasty Ban Liangs.

This coin is unconditionally guaranteed to be authentic