This is the famous 1976 bicentennial dollar struck in proof on a 40% silver clad planchet without the "S" mintmark. It was struck from the type II reverse which has the first "U" in "Unum" between the "LU" in "Pluribus".
The piece is unique and, according to the Coin World Comprehensive Catalog & Encyclopedia, was discovered in a cash register at Woodward and Lothrup Department store in Washington D.C. in mid-1977. It is said to weigh 383.25 grains instead of 350 grains, thus it was struck on a 40% silver planchet.
Only the single example is known and it was offered in Superior's 2/97 and Bowers and Merena's 9/02 sales.
Photo courtesy of Bowers and Merena.