I saw the glitch at the spiral in the pad and wondered about it; I thought it must have been fixed up digitally. And then I downloaded the 3kx5k version and saw the truth ... it's a paper patch!! Or, rather, three paper patches, apparently put in place before the work was photographed.
I got this print for my birthday and have just had it framed I'm so amazed I can't stop looking at it . It will take pride of place and should be a talking point for some time as friends and family admire it.