I think you're referring to MS-65. Please see the Related Question for more information.
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If that is a private mint coin, it is probably the weight of the coin.
You need to give the denomination and date of the coin. Also where you see the 77 at on the coin.
On an US coin it can mean "cents" or it can be the mint mark of the Charlotte Mint, which existed only from 1838 to 1861.
Numbers on a coin usually signify the year it was minted.
almost uncirculated