If the coin does have a mintmark it would be under the "DO" in dollar on the back of the coin.
Any mintmark will be found on the reverse of the coin under the wreath above the space between the D and O in "DOLLAR". Keep in mind that your coin could have no mintmark, in which case it would be minted in Philadelphia. If your coin has a mintmark, it will ether be a "CC", a "S" or an "O".
If the coin has a Mintmark it is on the back of the coin, above the letters DO in dollar.
The Monarchs head.
If it is a "s" quarter it will always have the S mintmark. You can find this mintmark usually on the back of the coin. The exact location will depend on the type of quarter.
The mintmark on the 1854 US quarter can be found on the reverse [tails] side of the coin beneath the eagle. The coin was minted at 2 different mints; Philadelphia and New Orleans. The letter "O" represents the New Orleans mint. The coins minted at Philadelphia do not have a mintmark.
Except on the 1964 issues where the mintmark was located on the reverse of the coin under the olive branch that the eagle is clutching. The mintmark is located on the front of the coin, below Kennedy's face and above the the space after the first 2 numbers of the year. Keep in mind that your coin may have no mintmark, in which case it was struck at the Philadelphia Mint.
The letter "D" under the date of a penny is the mintmark which indicates where the coin was minted. There are 4 operating mints in the USA. One , represented by the mintmark "D" is in Denver, Colorado. Another, represented by the mintmark "S" is in San Francisco, California. Another, represented by the mintmark "W" is in West Point, New York. The 4th, represented by the mintmark "P" is in Philadelphia, Pennsylvania. You will find these mintmarks on all coins struck in the USA today with the exception of the pennies which are produced at the Philadelphia mint. You will find no pennies with a mintmark "P". You will also find no coin minted in 1965, 1966 and 1967 with a mintmark. This is because during those years, to discourage coin collecting which some blamed for a coin shortage, none of the mint facilities in the US placed mintmarks on the coins they produced. This practice ended in 1967 and in 1968 mintmarks were again placed on US coins.
If your (walking liberty half dollar) coin is dated 1916 (and some 1917 coins) the mintmark is found underneath "In God we Trust" on the front of the coin. All other years the mintmark is found on the reverse of the coin to the left of the branch that the eagle is sitting on. Keep in mind that if your coin has no mintmark, it was minted in Philadelphia. If you have a silver eagle dollar which also uses the walking liberty design, the mintmark will be on the back under the olive branch that the eagle is holding. Bullion silver eagles do not contain mintmarks and could be minted from a number of different mints depending on the year the coin was issued.
512.00 is the spot price on the current market for 1 pound troy ounce fine silver morgan dollar proff dated 1878 but my advice is find a coin dealer or even better a private coin collector and your coin would be worth closer to 750.00
The S mintmark means it was made at the San Francisco Mint. It could be a copper-nickel proof or a 40% silver coin. Both have the same mintmark. Take it to a coin dealer or collector to find out which one it is.
Pictures of a specific coin are difficult to come by. Usually you should be able to find one on eBay, but the picture is withdrawn after the coin is sold. The only Five Pound coin issued in 1981 was a "Proof in gold FDC" coin.