The 3- and 4-speed output shafts both have 10 splines, but the splines are shaped and sized differently and the shafts are different diameters. Look again at the trans main shaft & front u-joint front yoke info in the chart. Model 1 is the 3-speed, and model 6 is the 4-speed (RPO on 3100 and 3600).
The 3100 is the only model with the full torque tube. I think the front shaft on the 3600 is enclosed like on the AD's. At least it is on the '41 according to the
\'41 shop manual.
