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.)