742 clutch type sure grip

69 dart 8 3/4 I have 28" yukon axles and evidently I need 28 3/16' axles.

I spun the splines off the end of the right side axle. The weird part is it only took a 1/4" of spline.

Question Is there really only 7/16 worth of spline?

It looks like there is two sets of splines in the dif. How does it work?