Two large Chinese characters Ban Liang ("Half an ounce"), no rims / Blank, no rims. 21.5mm, 1.61 grams. Gratzer/Fishman "The Early Round Coins of China" #13.134.

Interesting very small example.

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