Below are not MY words... I am not the expert here!
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The "two "M's" on top of each other gave it away. What you are looking at on the obverse of the coin is a small castle monogram with three arches. This represents the Segovia Mint in Spain. You didn't mention the coin's size but it is most likely a 1722 silver 2 Reales (about 2.6 cm in diameter or about 1 inch).
The coin was struck under Philip V of Spain and is not a cob, but a struck coin made on a "roller press" that gives the coin a slightly bent look. The assayer is "F". The letter "R" above the mint mark means Reales and if you could see to the right of the shield you would see the denomination "2".
The Shield is a study in heraldry and reflects the Hapsburg line of European rulers, and in the case of Philip V, the French Bourbons as well. The three fleur-de-lis in the center of the shield reflect the Bourbon lineage. Note the castles and lions (Castile and Leon), the rampant lion to the right, Brabant (Belgium), the diagonal lines to the lower left, Old Burgundy, the parallel lines at top right, Aragon. To the right of that, parallel lines are intersected by diagonals that within are two figures representing Naples and Sicily.
The reverse shows a cross and in each quadrant either a castle or Lion (Castile and Leon in Spain). A tressure or quatrefoil surrounds the cross. The date is at top.
These coins circulated widely in North America and were the models for U.S. coinage in the following century (Two reales=25 cents) and were legal tender in the United States until 1858. This series of coins are readily found in the numismatic market and come with mint marks from all over the New World as well as Spain. Value depends on grade. Low grade pieces as you described bring $10 or less.
