This pattern is a deliberate muling of two dies since each side has the inscription UNITED STATES OF AMERICA. The obverse die has the portrait of George Washington. The reverse is the regular die without rays, which was adopted in 1867 for the Shield Nickel.
This fascinating pattern muling was also double struck. There is a 5 degree rotation between the strikes on the obverse. The reverse shows a 45 degree rotation between the two strikes.
This is one of two known mulings of this pattern that are double struck. The other known example shows an 80 degree rotation on the reverse between the strikes.
There are many different Nickel patterns struck in 1866 using different die combinations. 1866 is considered one of the most fascinating years in the history of U.S. patterns.