I have never seen 6 V brake magnets. And your pic looks nearly like the 12 V system that I have on my '66 Overlander. The shoes are a little different and you have leaf springs and mine does not.
However, the round magnets that you have look identical to my unit. And it looks like the magnet wear is down to the screw heads. Time to replace them anyway.
Another thing that I will point out. The major auto manufacturers stopped using 6V electrical systems by 1958
. Given that Airstream is a premium trailer builder and some what ahead of the curve, I would say the original brakes were 12V
. Also I would suggest that in 45 years the brake magnets on your trailer have been replaced at least once before. Most likely they were replaced with 12 V magnets.
Just my musings. No real facts other than the visual.