I bought this coin, thinking it was Vietnam 10 van. The characters don't match anything I have found for that denomination, and I now doubt it is Vietnamese at all, but Chinese or Korean.
Reading the front it does say "thanh Thai thong bao" but the other side is throwing me with only having the two characters, I can't find any matching it.
No, it is an Annam (Viet-nam) cash. An old Yeoman catalog I have says Y2. It's buried in KM because it is listed under "Viet nam - French Protectorate" in the 1801-1900 catalog. Here is another listing. http://art-hanoi.com/toda/22.html