It is a Class III doubled die- Design Hub Doubling. This class occurs when a die is hubbed(impressed with a positive die) with two different design types. The die used to strike this coin was first hubbed with the large date design and the second hubbing was done with the small date design. To make matters even more confusing, a proof Large/Small Date exists as well as a 1960-D Small/Large Date.
This class is also responsible for the 20th century overdates- 1918/7-D Buffalo 5c, 1943/2-P Jefferson 5c, 1942/1-P and 1942/1-D Mercury 10c, and 1918/7-S Standing Liberty 25c.