On Harrington's model in which Separation holds but Reduction fails at the 3rd projective level, and on some related models of Sami

In a handwtitten note of 1975, Leo Harrington sketched a construction of a model of ZFC (no large cardinals or anything beyond ZFC!) in which ${\mathbf{\Pi }}_3^1$-Separation holds but ${\mathbf{\Sigma }}_3^1$-Reduction fails. The result has never appeared in a journal or book publication except for a few of old references.