The daily digit is the low temperature on this day in 1964 at Waterloo.
From the National Weather Service: March 27,1964: Unusually cold late March weather settled over Iowa on March 26-27 following a winter storm that had produced snow, sleet, and freezing rain across most of the state. On the morning of the 27th the coldest readings were reported across northern Iowa where temperatures fell to as low as -2 F at Fayette, -3 F at Decorah and Storm Lake, -4 F at New Hampton, -5 F at Waterloo where the temperature has only once fallen below zero on a later date, and -8 F at Saratoga in Howard County.
The map below is from the morning of March 27, 1964
Going back the stat for Waterloo: The only other time the temperature was colder than zero, after March 27, was when the low was -4 on April 6, 1982.