Today the factorial is universally denoted with an exclamation point, but it wasn't always so. Gauss wrote *n*! as Π(*n*), for instance. A bizarre-looking "corner" notation for factorials competed with ! in the 19th and early 20th century before eventually dying off. See a question about the corner notation on math.stackexchange here that uses the notation too. In a 1997 sci.math post that can be seen here the
response by William Waterhouse discusses when the corner and ! notations were introduced and became popular.

I first saw the corner notation in the pages of Hall & Knight's "Higher Algebra". Here is a scan from two pages. On page 116 the notation is introduced just before the start of section 142 and the authors say ! is "sometimes" used.

From a comment on another math.stackexchange post here I learned that even Hilbert used corner notation in one of his papers. A screenshot from the second page of a paper of Hilbert from 1894 is below, with corner notation in the middle displayed equation. (The full article is available here.)

Obviously the people who wrote papers or books with the corner notation for factorials were also using it in their writing, but I wonder if there is any other photograph showing a person in the act of such writing. If you find one, let me know.