Some info from Coin Facts.
The 1938-D/S Buffalo nickel variety was first discovered circa 1961 by Robert Kerr and C.G. Langworthy. The two collectors submitted their findings to the then new "Coin World" weekly publication and subsequently no less a numismatic luminary than Q. David Bowers confirmed the authenticity of the over-mintmark. Since no Buffalo nickels were minted in San Francisco in 1938, it seems likely that some S-mintmarked dies were deliberately repunched at the Denver Mint. There are several variations of this over-mintmark and only the "strong" variety commands a noticeable premium. While quite interesting, the 1938-D/S is not all that rare. And like the regular mint mark 1938-D, this issue often comes well struck with great luster.