Tons of original engineering drawings for the P-51.
When the factory gave them to the current owner in 1988 a lot of the were wet from a broken pipe and had to be dried out.
Fortunately most were vellum with graphite pencil drawings and sketches which handle water pretty well.
These are rare because most manufacturers transferred everything to microfilm and destroyed the originals in order to save space.
Unfortunately for some reason microfilm has trouble with vellum.
Digital doesn't have this problem.