The inner shaft is the input. 3 different shafts. Standard 67 and up non lock up, 24 spline normal for available converters, 66 and earlier 19 spline, and lock up 23 spline. The outer splined portion is the stator support. 66 and earlier stator support inner diameter is smaller and will not accommodate a later 23/24 spline shaft. The earlier shafts and lock up shafts can be swapped to fit the 24 spline shaft if the proper reaction shaft support and matching high gear drum are used.
Doug