The Watermark decals appeared at about the same time the book ‘Spitfire – The Canadians’ did.
http://www.largescaleplanes.com/reviews/review.php?rid=349
The decals cover many of the same schemes illustrated in the books and while I never asked the question I always assumed there was some tie-up between them. Friends of mine in Canada rated the books highly and thought well of the major contributors, so I’ve assumed the content to be reliable.