Eric Webb put together ONI-values from 1865-2017 and from that statistics, I see that there was another event with similar behaviour as the 1964-1966 period. If Webbs numbers are correct, and I don't doubt they are despite the higher uncertainity of older data prior to 1950, the 1902-1904 period first saw a strong El Niño with a peak at +1,67C in fall (SON) 1902 followed by a weak La Niña by fall 1903 peaking at -0,92C by DJF. This was followed by a new El Niño by fall 1904 which aside of being moderate (peaking at +1,36C by ASO 1905) also was unusually longlived and didn't decayed until spring 1906. Link to Webbs statistics: http://weatheradvance.com/home/weather/weatheradvance.com/wp-content/uploads/2016/07/Ensemble-Oceanic-NINO-Index-ENS-ONI-Raw-Data-1865-2016-2.txt