Any cash coin experts who can tell me more about this? I don't think it's Chinese and my first guess is Vietnam (Annam). If not that, Korea or Japan?
Everyone is correct, this is from Korea, but some details are incorrect. I am using Cast Korea Coins and Charms by Wybrand Op den Velde and David Hartill for a reference here. Hopefully I have this ID'd correctly. Your coin is dated 1883; it's reference number is 21.1B.3. The obverse reads "Sang Pyong Tong Bo" from top-bottom-right-left. The reverse reads "Chon Tang O, 3". The Chon is the mint, known as Chonhwanguk or the Central Government Mint. It was located in Seoul. The Tang O means equal to five; the denomination is known as 5 Mun, more than most. The 3 below is a variable number with this type ranging from 1 to 12, I suppose as some control mark or officina number or something of that sort. Hope this helps!
Looks like your question has already been answered, but in cast this is useful to someone later: David Hartill's "Cast Chinese Coins" has a guide to ID coins by the obverse characters which includes coins from Japan, Korea and Annam. It also has calligraphic versions of the characters such as the seal script versions where applicable. For example, your coin is listed under 11 strokes: 常平 Sang Pyong Korea
