Centre d'histoire sociale du XXe siècle, Françoise Blum, Sylvie Le Dantec, André Kaspi, Les vies de Pierre Naville, 2007, p. 79
nytimes.com
query.nytimes.com
For an instance of war-time naming of this gun as "Big Bertha", see Paris again Shelled by Long-Range GunThe New York Times, August 6, 1918, page 3 online abstract