Ran into this in my 1965 Dart w/ 10" drums. I was just removing to de-rust and paint parts and install new ball joints. I couldn't get one of the two bolts out. A nut had secured the drum backing plate, so I ASSumed the bolt was in a thru hole in the spindle so beat it out with a hammer, figuring just stuck in the hole with rust. Surprise, it was screwed into a tapped hole in the spindle, so my hammering stripped out the threads. I was then educated on the other side, so didn't molest its threads. When re-assembling, I couldn't ascertain which spindle was left and right, even after staring at many web photos. The PN's didn't seem correct (odd vs even), as best I could make them out in the forgings. I also could remember which side I beat the bolt out on (vaguely recall left) since the buggered hole told me, if threaded on the front hole (your problem bolt is aft one). Anyway, it seemed they needed to be so the steering arm dropped down going aft for the steering parts to fit. No problem with the 9" drum "gooseneck" spindle (obvious in photos). Pretty sure that bolt was RH thread since I would have remembered if the outer nut had to be unscrewed backwards.
My guess is that aft hole in the spindle isn't threaded. As others suggest, that might be a non-factory bolt which was perhaps jammed into the hole. In my case, I wasn't concerned with the stripped threads in my spindle since the outer nut secures every thing, and I don't think later spindles had a threaded hole. It was likely used to pre-assemble parts at the factory. My recollection is that the drum bolts are 7/16", while K-H disk ball-joints use 1/2" bolts, but perhaps it is 1/2" and 9/16". I just recall that K-H uses larger bolts. There are posts about that which you can search.
