I am wondering if they weren’t covered in wax or some other water barrier. Or perhaps the protective disc was welded at the seams. Seems unlikely given the era

I am wondering if they weren’t covered in wax or some other water barrier. Or perhaps the protective disc was welded at the seams. Seems unlikely given the era and the extra expense involved. It also mentions that they were known to fail, so one could assume exposure to the elements played a part.