From January 1959 until July 1963, the 5 units or more category also includes 2 to 4 unit structures.
Source: US. Bureau of the Census
Release: New Residential Construction
US. Bureau of the Census, Privately Owned Housing Starts: 5-Unit Structures or More [HOUST5F], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/HOUST5F/, September 3, 2015.