The most up-to-date version is known as Kaplan Extended SST v2, which combines statistically infilled grids of UK Met Office SSTs for 1856-1981 with a statistically reduced and coarser resolution version of Smith and Reynolds Optimally Interpolated (OI) SSTs for 1981 to present. Those desiring only recent data should use another product.
Least squares procedures of optimal estimation, when applied to gappy and erratic data, result in the solutions which predominantly project onto the most energetic patterns of a priori error covariance. This property of the solution allows to combine the classical least squares technique with the approach of a space reduction in order to develop a computationally effective procedure of objective analysis for observed historical climate data. (Such data are characterized by comparatively precise observations and good coverage in the last few decades, and poor observational coverage prior.) An important aspect of our approach is that it also produces verifiable error bars for analyzed values. For details of the technique see Kaplan et al. 1997, for a simple qualitative introduction see Kaplan et al. 2001.
Here we applied the reduced space optimal smoother technique to the U.K. Meteorological Office observational data set of historical sea surface temperatures (SST) (Bottomley et al 1990; Parker et al 1994). The primary difference between the British data set and the COADS SST is that the former was corrected for the systematic biases in bucket measurements of SST before 1940s (Folland and Parker, 1995). The details of our analysis (http://ingrid.ldgo.columbia.edu/SOURCES/.KAPLAN/.RSA_MOHSST5.cuf/.OS/.ssta/) and its verification are given in Kaplan et al. 1998, but there are a few points I'd like to make here:
Alexy Kaplan, Columbia University, May, 2001###
Kaplan, Alexy & National Center for Atmospheric Research Staff (Eds). Last modified 10 Feb 2017. "The Climate Data Guide: Kaplan Sea Surface Temperature Anomalies." Retrieved from https://climatedataguide.ucar.edu/climate-data/kaplan-sea-surface-temperature-anomalies.