The 52 inch 3 cylinder motor was only offered with the boxy 'square' cowling. Mercury switched to the 'round' cowling design in 200x, after they had already moved away from 52 inch motors. Thus, a square cowling motor may have a 52 or 59 inch powerhead but all of the round cowling 3 cylinder motors are 59 inch.
Along with the overal look of the motors, the clamp was changed.
Mids came in two lengths, 15" (short) and 20" (long). They also changed from square to round cowling.
Most of the 52/59 inch motors came with 1.83:1 gear ratio lower units. All are F N R shiftable. The prop shaft is 13 spline and has a hub diameter of 3.x inches. The only exception was square cowling 60 HP motors, that came with 1.64:1 gear ratios.