The 1976 (Type 2) Proof Bicentennial Eisenhower Dollar coin without an "S" mint mark is probably the rarest non-error modern coin in existence. All other known 1976 Bicentennial Proof Eisenhower dollars bear an "S" mint mark as they were all produced at the San Francisco Mint. This Eisenhower coin was produced in Pennsylvania and made of Silver.
The unique 1976 No S Proof Eisenhower Dollar is graded PCGS PR66DCAM.

On August 12, 1974 the three winning designers of the Bicentennial coins were allowed to strike trial proof coins at the Philadelphia Mint, well before the Bicentennial coins went out to the public. Since these trial pieces were struck at the Philadelphia Mint, they did not contain the "S" mint mark. These trial strikes without "S" mint marks were then sent to Bal Harbour, Florida to be displayed at the ANA Convention