Another Chinese cash mystery [résolu]

Hello friends,

I swear I tried to ID this one but again I failed miserably.
I think there's a lack of internet source for non-Chinese readers to allow cash coins ID.


Well, this coin was a pitch black round thing, I ruined it leaving on coca cola for few days, but now at least we see something. The obverse is blank (or should be the reverse?).
Size: 22mm

Any light of what is written or what it is?
Thank you for any response.
Geison

It looks a charm (+tong bao)of the 20th century. But i didn't find the characters

Vietnam Empire
啟定通寶 - Khải Định Thong Bao

https://en.numista.com/catalogue/pieces11309.html

Wow, thank you masters!!
I am very glad with this coin now =D

Solved!