It’s not uncommon to find penciled notes on the materials I’m rehousing. Usually it’s something like the date or names of correspondents (expanded, clarified, or simply rendered more legible). Perfectly reasonable (though not something I’ve ever needed or brought myself to do) and usually transparent, but sometimes delightful.
A letter from Charles Lyell to John Stevens Henslow is a particularly entertaining example. “Botanist” is penciled beside Henslow’s name, “geologist” at the bottom of the page. A redundant “1832” is added next to the “Feb 15” following the text (itself a redundancy on the part of the author, as he had written out the full date at the top of the letter). There are a couple erasures (“early” and “15”) at the bottom of the page.
But best of all is the purely editorial comment penciled into the lower left-hand corner: “very nice letter.”