Two large Chinese characters Ban Liang ("Half an ounce"), outer rim / Flat, no rims. 24mm, 2.15 grams. Gratzer/Fishman "The Early Round Coins of China" #13.148; BM Chinese coins (Poole) #256ff; Hartill #7.29. SKU T1828-50079

"Shizi" form of the "liang" character, with the lower part of "liang" is like a sideways "E". 

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