The smooth area at the front of the splines was to accommodate a seal on the output shaft (seen on right in pic below) that was used for the first years of the slip yoke.
Thx for the pic, would it be possible to get a measurement of how long that splined section is? My guess would be only 3"
Yes, the splines on the shaft are 3" long. On a typical 727 factory slip yoke, the bore is 6 1/2" deep and the un-splined area is 1 1/2" deep, so there are 5" of splines internally.
