The PDF itself is about the evolution of the Bianchi 9R that was featured in Clint Eastwood's 1977 film "The Gauntlet". But because the original post mentions a friend's new X-15, and I've acquired one in pristine condition myself recently, below is about the X-15.
The X-15, named for the rocket plane of 1959, appeared first in JB & NP's 1964 catalog as Safari Ltd:
Above and below is one you'll never see again because the X-15 you know today was made using the 'ears' of the 1930s Clark shoulder holster (sags open for holstering) vs the closed sewing shown here (can catch on the hammer spur). It is under JB's "Combat Action" label that preceded his association w/ NP:
Below, a VERY early X-15 with the 'ears' of the Clark. The boys made the springs themselves and one of them told me the story of cutting them to length using a cold chisel and sending spears of spring wire flying across the shop!
Super rare, too, below: for the briefest of moments after some appeared as Safari Ltd., below is one while NP had changed the company's name to Safariland; the model was then dropped in favor of his elastic loaded shoulder holsters that were both inverted and vertical:
"Holstory -- Gunleather of the Twentieth Century -- the Second Edition" is available at www.holstory.com and is posted to you from inside the USA by the printer itself.