I'm the first to say "I'm dumb" so much so that I put it in the title haha. I had no idea about unc, unf etc etc, just haven't been around this kind of stuff.
And I only got hold of some UNC bolts today and to my surprise they fit the same as the metric ones I got. So after a closer look I reckon the thread is flogged out, but flogged out the same on all 6 nuts.
1/2" UNC it is and just crank em through the last two rounds if thread with a spring washer to help keep it tight

FanTapstic!