Two Chinese characters Ban Liang ("Half an ounce"), outer rim / Flat, no rims. 23.5mm, 2.80 grams. Gratzer/Fishman "The Early Round Coins of China" #13.39; BM Chinese coins (Poole) #256ff; Hartill #16. 

"Renzi" liang describes the shape of the character "liang". It is a distinct calligraphic variety of 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 reigns of Emperors Wen (180-157 BC) or Jing (157-141 BC), the date of the casting is ca.175-140 BC.

This coin is unconditionally guaranteed to be authentic