To the best of my knowledge, the factory did not use nuts. If I am correct about that, I guess a past owner of the convertible must have stripped the threads also, just like me.
Seems like a reasonable conclusion.
One solution may be to use a square nut instead of a hex. The only advantage this will have is IF its the right size the retainer will prevent it from turning as you finish tightening the screw.
Just so you know, the black output wire from the alternator does not go to the ammeter. It goes to the main splice. From the main splice one branch connects to the battery through the ammeter.
On a '65 there should be a 16 ga fusible link somewhere between the battery and the bulkhead. Maybe right at the bulkhead - factory would be Blue. Some replacements are different colors. I mention it because in the first photo I see a small red wire. For a convertible especially a good connection, and a good crimp there, in addition to the fusible link are all worth the extra effort.
