I downloaded the zipped file of those pages. It was at the bottom of the above-mentioned page. The smudge is not on that copy. The zipped file consists of all the pages as a .gif, or graphical file, but possibly still readable when printed.
There's a lack of clarity and printing from the .gif files will add to that, being another generation away from the original. However, when I get a chance, I'll try scanning and OCRing the pages and let you know if I have success. In the meantime, I've loaded the zip file onto the server and you can access it here:
Sargent Notes
By the way, you'll need WinZip on your system to uncompress the zipped files and access them. If you don't have WinZip, a trial version is downloadable for free at
www.winzip.com
What the paragraph says that has the smudge is:
"He then with a bit of charcoal placed the head with no more than a few careful lines over which he pressed a rag, so that it was a perfectly clean grayish colored canvas (which he preferred) faintly showing where the lines had been."