Best we can tell, a u-joint failed in the past. Rather than fix the shaft, the last person to put u-joints in just shoved one in. It was likely okay based on the surface area the cap could contact but why risk it?

So, the slip yoke is 7260. The shaft is now 1310 on the trans yoke end so I have a 7260 to 1310 u-joint. The other end is 7260 but the yoke on the 489 case diff is 7290 so I have a 7260 to 7290 u-joint there. The diameter of the drive shaft was 3.5 or 3.25.... can't remember. Either way, they could get one of those two diameter ends with the 7260 and I had the other.

Regardless, hot tanked, repaired, painted, new u-joints, and balanced for a buck fifty. Not bad in Atlanta I'd guess.