These are obverse and reverse die trials for the 1904 Lewis & Clark Exposition commemorative gold dollars struck on a single piece of cardboard.
Pollock gave this trial two numbers, P3518 for the obverse and P3521 for the reverse not knowing that both obverse and reverse were on the same piece of cardboard. They really should only have one number.
To my knowledge, they have never before been seen by the collecting fraternity.
This is believed to have come from the Nagy collection and was given to the Smithsonian by Stacks in 1967.
Photo courtesy of the National Numismatic Collection of the Smithsonian Institution.