Hundreds of enthusiasts, collectors, investors and dealers attended the January 24 sale in New York to see the 1794 Flowing Hair silver dollar put up for auction as part of the Cardinal Collection of early silver dollars.
At lot 13094, what was billed as the "one of the greatest American numismatic landmarks" was the final item in the sale and came without a published estimate.
Its significance is that it is accepted as the finest known example of its kind, coming from the earliest striking of silver dollars, with its condition pointing to it being the very first coin minted in that issue.
"Quite Possibly The Very First Silver Dollar Struck," said the catalogue.
"A close study of its characteristics suggests that it may well be the first specimen struck of the first year of the silver dollar, and was carefully preserved for posterity," said the auctioneers.
In achieving the price, Stack's Bowers broke the record they set in partnership with Sotheby's in 2002 for a 1933 Double Eagle gold $20, which took $6.6m (then £4.55m) against a $3m-4m estimate.
The buyer's premium was 17.5%.