Here's pictures of the different versions of gears. Early Scout with 30-40-77 teeth count, 101 Scout and 22-31 Chief has the same gearing 33-36-84, but different shaft diameter, and later Chief and Std Scout with 39-36-84. I don't know the shaft diameter of the 39 cog gear.
33 cog Chief gears might have had a larger diam of the shaft taper in years pre -28, and same taper as the 101 in the same years, but I am not sure.
Courtesy of Ralf Spindler, shown is a Chief 33 cog gear on a Scout shaft.
In order to fit the 39 cog gear without changing the 36 cog intermediate gear, the case is a one, two, three year special...before the chain transmission was implemented.
I think the idea with the 39 cog is to speed up the gearbox gears in order to even up and lower the thrust impulses from the engine, and make the gears in the gearbox last longer...And also compensate for the large chain sprocket needed on the brake drum..