Jaime Hernandez: In 1939, the Mint struck 1939 Jefferson nickels with two different reverse designs. The first design is referred to as a "Reverse of 1938." The second design is referred to as, a "Reverse of 1940."
The transition took place due to minor flaws on the reverse design. The original design had weak and curved step details on the Monticello building located on the reverse design of the coin. The easiest place to notice the curving on the steps is on the center below the last step on the Monticello building, where it curves significantly once compared to the new design.
Therefore, on the new design or the "1939 Reverse of 1940" the design displays sharper details and a straighter set of steps on the Monticello building on the reverse of the coin.
The 1939 Jefferson nickel with a Reverse of 1938 must have been produced for a very short period of time, since it is the scarcer of the two different reverse varieties.