Singer used the 15-xx on both Industrial and Domestic machines, one of the reasons for the confusion for sellers saying the Model 15 is industrial - when they really have a domestic production machine. And sometimes Singer used the same -xx for both the domestic and industrial version - just put heavy duty insides in one of them.
The 15 Class was made from 1895-1957
Yours in is this "group" BTW- the 15-86 is in an earlier age group
Produced between 1933-1956 (though it looks like Singer stopped production on the treadle/hand style before the electric)
88 is treadle (spoked flywheel)
89 is hand crank (spoked flywheel)
90 is electric belt motor (solid flywheel)
91 is electric potted motor
More often than not, for any given time frame of production, the -XX numbers differentiation between power sources, and not any other options.