Thank you Chasa. I don't have that reference either. The best I had were the articles in the US Specialist, which documented a lot of the repairs.
I think it looks like ink starvation?
Peter, No. Having learned many years ago of the crudeness of many of the precancel plate repairs, this one jumped out at me. Consider a different view squished down like the 1851-57 folks do. Normal at left, repaired plate on the right ...