Huh! So this is the only copy of Locus Solus that I have right now – I really wish I could compare the older English edition to see what they did. The François Caradec biography is what I’m taking as my source:

For the printing of Locus Solus, Roussel “went so far as to create a special typographical character, a sort of tonic accent apparently lifted from some Oriental language, to give the correct form to the highly complex name of one of his characters.” [Saint-Rál, Le Gaulois, 17 February 1923.] This relates to two letters with diacritical marks that do not appear in the fonts of the Latin alphabet (and are thus unpronounceable in French) for the cat whose name means “plaything” in Siamese: Khóng-dek-lèn. [see note below] These characters were specially forged. As the name appears sixteen times, and occasionally in several places on the same page, the character could not have been bodged up in the printing-room, but several copies of it must have been produced. Roussel really stopped at nothing! In Locus Solus there are also specially engraved musical scores. For the typeface of the title of Andante a Normande font was used, which stands out from the Elzevir used by Lemerre for the rest of the text. (trans. Ian Monk; p. 155)

Here’s a scan of the note I left out, which includes what I assumed was a scan of the original printing:

To me, that looks like a breve under a macron; however, the image is grainy. Would love to see more on this; the Mark Ford bio doesn’t include very much.