Two large Chinese characters Ban Liang ("Half an ounce"), vertical Er ("2") below hole, outer rim / Flat, no rims. 23mm, 1.65 grams. Gratzer/Fishman "The Early Round Coins of China" #13.207 var (not listed with an outer rim); BM Chinese coins (Poole) #-; Hartill #7.30.

Rare with the additional "Er" on obverse.

"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