I believe the coin is not from Mexico; rather from Guatemala. The "G" you see is part of "NG" the mint mark of Guatemala. The "M" is not the mint mark of Mexico but rather the Assayer Mark for Manuel Eusebio Sanchez. The coin is 89.6% silver.
The full description would read:
Obverse : Laureate, armored and draped bust, facing right; "CAROLUS·IIII·DEI·GRATIA·"; (date) below between dots.
Reverse : Crowned Arms, between pillars; "HISPAN·ET IND·R·"; ·NG· at left and ·M· above left.
Don.......