Here are the first few pages of the original 1928 Andrieu Freres edition of Paul Jeanjean's Book of 18 Etudes de Perfectionnement pour Clarinette. Richard Hawkins kindly scanned these from the Oberlin library, which owns Gustave Langenus' personal signed copy. (To my clarinetist readers: please be sure to respond to my quandary below)
It shows the notations Jeanjean attached to the mysterious footnote numbers above certain measures throughout the book, and which footnotes themselves are missing from the Alfred edition available in the US. As you can see, they are simply clarifications of the various "modern" chords he used in these advanced etudes.
I hope most of you can open and see the PDF linked above. It takes a few seconds to upload. And it may appear blank until you scroll down to the printed part.
Now for the quandary. Please scroll down to the second page (which is a the first page of the second etude). Look at the last measure of the second to last line. You can see Langenus marked a G sharp, which is in the key signature and so should be G sharp. Now look at the notation #6 at the bottom of the page. It clearly shows a chord with a G natural, (there are not key signatures for the notations, and every accidental is marked). Jeanjean even states that the first interval should be a major 2nd, (G natural to A natural) and that there should be a tritone, which would require a G natural to the C sharp marked.
Let me know what you think. I say the part should have been printed G natural, and Langenus didn't pick up on the notes below. Or Langenus natural sign looks very much like a sharp. The pencil mark is not perfectly clear, but it looks like a sharp to me.