I have worked on a lot of 56 F-100 and the drive shaft goes directly into a yoke that fits over the transmission (only familiar with manual transmissions). The F-100 I have experience with all have the emergency brake applied from inside the rear wheel drum. On the F-250 I have a different set up. The drive shaft is two piece. The first section off the transmission is short and has a drum brake around the drive shaft to apply the emergency brake. That short drive shaft is connected to a longer drive shaft that connects to the rear end. On the end of the long transmission that hooks to the short drive shaft, there is a metal threaded cap that screws over the end of the yoke. It has the same spline pattern as the spline, but is not all steel. At the rear of the metal spline tooth pattern, there appears to be some kind of "fiber" material. It appears this fitting can be rotated after the drive shaft is inserted into the yoke, but I have no idea what the purpose of the fitting is or how to adjust it once the drive shaft is together. Can anyone explain what this is?

 

That is a grease seal. The splines need lubrication and the seal keeps the grease in and dirt out. Tighten it hand tight.

 

I'm pretty sure your talking about the slip yolk but a picture always helps:

 

I still have not mastered pictures, but I do believe "grease seal" nailed it. I have never seen anything like this on the F-100's.

 

The long shaft slides in and out at the splined section to account for the axle moving up and down. Keeping the splines greased is really important.

 

I will have to see if I can find a replacement seal. The cap has metal splines that match the pattern of the splines it screws into, and some kind of other material besides steel behind that which also have the same spline pattern as the steel.

 

The originals were nothing more than thick cork. Very easy to make a seal from gasket material or leather. It would probably seal quite well if you wrapped quite a few turns of string or cord between the split washer and the spline. Even a new seal may leak some grease when you purge the spline fitting.

 

The part you need appears to be B3D-4820-A, Dust Cap Seal, and Dennis Carpenter appears to have them.