Slight Machine Doubling and die wear on the mintmark. Note it is normal size and it took a hit on the upper serif. If it were a RPM, the lower serif would also be affected. Note on this page how all the RPM's have enlarged devices: http://www.varietyvista.com/05%20JN...%201964D.htm When the mintmark is normal size, it can't be a RPM, unless the serifs were affected like a tilted RPM. (Whats a Tilted RPM?)
Tilted/rotated RPMs are seen more on the higher numbers of the 1960-D cents. (More common to find and often not listed) http://www.varietyvista.com/02b%20L...(61-113).htm Again note that the RPMs enlarge the devices, but not as much on the Tilted/rotated RPMs.
CoopHome: What is a tilted RPM? One of the different RPM directions