OK, I am going through all my random foreign coins and trying to get them all identified and put in a book. Here is a coin that I can't identify. I know that it is from Mexico and is approximately 20mm in diameter. The coin appears to be silver. Clearly it is heavily worn. I would like to confirm that this is Mexican. Then, please help me identify the denomination, a range of years it was minted in, and it's composition. Thanks!!!