it was probably corrosion but dam if I can see anything wrong they look like new to me. I guess you have to really look to notice this...The wipers on the fork seals and the dust caps are quite effective in removing oil from the exposed part of your forks. In sitting unused for 30 years, any thin layer of oil on the tubes may have been insufficient to protect it from rust that would lead to pitting. Sand or small rocks hitting the tubes while riding can give rust a place to start. But with only 850 miles you have to wonder how any amount of dirt hit those tubes.