They were riveted on with semi-tubular rivets. I have no idea whether repop fenders come with the brace riveted on or with holes (I'm guessing they have holes at least).
As far as keeping the carriage bolts from spinning, they'll not do that if you keep pressure on the head and they're new with new nuts. Or you can file the hole to a square shape.
PM me if you need some rivets.
