Cornelius Bryan was not illiterate. He signed his will with a mark (coat of arms usually), that is an ancient method of signature, going back to times of nobility.
"However, a mark or symbol such as the image of a coat of arms or other graphic image that is drawn by hand, stamped or embossed onto a document would constitute a signature if applied by its authorized user with intent to serve as a signature. For instance, in ancient times, members of the clergy, the nobility and other persons of wealth often used the impressions of the unique images on their signet rings as their signatures. Today, the most common analogy is the handmade “X.” And you do not have to be illiterate or physically unable to write in order to adopt a mark or symbol as a signature.".
X
Source - https://www.nationalnotary.org/notary-bulletin/blog/2018/08/a-notar...