The nut (B6C-7077-A) is called a "cap" by Ford. Does your truck have a grease seal, and if so...is it within the cap itself?

There is no parts catalog listing for a seal, except for F500 and larger trucks. So it might be included with the cap.

There is also no parts catalog listing for a Zerk fitting on the slip yoke.

My 1965 F100 used the same slip yoke (C5TZ-4841-F) as the OP's truck, and as I recall, it didn't have a Zerk fitting or a seal (I bought the truck new-owned it thru 2009).

But memories tend to fade and the '64/72 truck parts catalog, considering it has 6,909 pages of text & illustrations, with 10's of 1000's of part numbers listed, may not always be 100% correct.

 

ND, there was no grease seal in place when last disassembled. The "cap" fits around the outer flutes of the shaft and does a good job of keeping the grease in the yoke and dirt out. The OP's picture shows rusty threads on the yoke which means the cap has not been in place for some time. The zerk is located close to the u-joint.

Kurt

 

I had the same problem with mine. If you remove that yoke and measure the shaft (how much of the shaft sticks out past the support bearing) that will give you a good idea. Mine sticks out 2.75 inches, and the yoke only slipped on about 1/2 inch! Not enough! If you hit bumps and the rear of the truck goes up & down it came come past that (and remember with only 1/2 inch of travel before it slips off the end, it does not have to slide out very far before it fails). My failed yesterday, luckily I was only going about 30 when it slipped off. Bad design. Plenty of room for a longer slip yoke, I am googling now, I might buy C9Tz-4841-A which is 7.75 inches, vs the original C5Tz-4841-F which is 6 inches (correct way to measure is center of u-joint hole to tip of yoke).