Diametral Pitch was used in BS 436: 1940 Machine Cut Gears.
I also have a copy of Machine Cut Gears from David Brown dated 1921 which refers to Diametral Pitch. The introduction in the book says that it is a reprint of their 1903 book.
I have nothing earlier, but the term seems to have been in use for a long time.
But I agree with Duncan, we all know what you mean, most of the time we just use the symbol DP and not worry what it stands for.
Neil
Edited By Neil A on 20/06/2017 14:06:43