Two Chinese characters Ban Liang ("Half an ounce"), outer rim / Flat, no rims. 24mm, 2.07 grams. Gratzer/Fishman "The Early Round Coins of China" #13.65; BM Chinese coins (Poole) #256ff; Hartill #7.29 var. SKU T1856-50057
"Shanzi" liang describes the shape of the character "liang". It is a distinct calligraphic variety of character "liang", used mostly in the Han period. Rare with a rim.
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