If you can, find a copy, the "SHOOT TO LIVE - Presenting the Johnson Method of Musketry Coaching as adopted by the
Canadian
Army" published in 1945. It does not cover the sniping rifle variant but it does cover how to hold, shoot and zero the No. 4 Mk. I* rifle.
For those of you who have not trained with the old FN (C1A1, L1A1) or with Lee-Enfields in .303, this book can save you a LOT of pain. Modern soldiers place their rifles very high on the shoulder, on top of the collar bone, presumably because they are used to working around the body armour and as they ALL (regardless of country) seem to do it, I expect that it is now official procedure. I have seen photos of Regular Army instructors teaching the Canadian Rangers to shoot this way (without the armour) using old Lee-Enfield .303" rifles. Big mistake! The above book shows you how to correctly place a Lee-Enfield butt onto the muscle BELOW the collar bone. (see pp. 56-57 etc.)