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 the 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.)

A more recent book than Hall and Knight with corner factorial notation is Eisenhart's *Introduction to Differential Geometry*, from 1947 (corrected edition). Part of one page is shown below.

In 2018, while inside the math department at Colorado State University, I saw several photographs on the wall with scenes from the campus long ago (it was founded in 1870). One photograph in particular astonished me: a person using the corner notation on the blackboard.

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.