Comment by pseudohadamard

I've had a quick look and it's mentioned in a number of places, Matthew Aid's "The Secret Sentry" mentions it briefly, Bamford has a page or two in "Body of Secrets", and for honorable mention Jeffrey Richelson's "Wizards of Langley" doesn't have it but does cover the entire KH series across a span of around 250 pages. It may be in a few more books but it'd be a pile to go through.