The one casting number that can tell you whether the trans is 23 spline or 18 spline in 68 and 69 only, on B/C bodies, is the tailshaft casting number .... ASSuMEing the trans has never been monkeyed with ... the tailshaft casting is different between the 18 spline and 23 spline because of the size of the 308 bearing and only the Hemi 4 speed in 68 and 69 had 308 bearings .