Ron Guth: The 1949-D over S Nickel is a well-known, popular variety that shows an S underneath the D mintmark on the reverse. How can this happen? Why would a punch for an S mintmark be used in Denver? The answer is simple -- all of the dies, including those for all of the branch mints, were made at the Philadelphia Mint. There in Pennsylvania, Mint engravers prepped dies for the Denver Mint by punching a D mintmark into the reverse die. In the case of the 1949-D over S Nickel, two scenarios suggest and answer to our dilemma. Either the Over-mintmark was an error, where the engraver started with the wrong mintmark punch, or there was a surplus of leftover dies that had already been punched with an S mintmark. We suspect the former.
PCGS has certified over 300 1949-D over D Nickels, the vast majority of which are in MS-65, and only a few of which show full steps. The finest example certified by PCGS is a single PCGS MS-67.