Rare Rare Nickel !

In 1939 the reverse was modified (the steps) to try and produce a fuller detail. Both reverse styles were used in 1939 (old 1938 & new 1940) at all three mints including the proof. The 1940 proof is also said to carry the 38 reverse but is rare. This is what I understand it to be. Maybe someone else can add some to it.

The title, A guide book of Buffalo and Jefferson nickels tells about it. Written by Q. David Bowers and published by Whitman. Very good book.

Cherry Pickers Guide Lists them both . The vast majority of 1939 proof Nickels have the strong steps design of 1940 so the "wavy steps" of 1939 are tough to find there. Business strikes from all three mints in 1939 can easily be found with both reverses. Almost all proofs of 1940 have the strong step design, the wavy weak steps of 1939 are very rare on those. There are no known business strikes from 1940 with the wavy step design.

This info is from the Cherry Pickers Guide.