Big Berta is a colossal cannon present at the siege of Paris in 1870, in Uchronic Paris.
The historical Big Bertha is a German howitzer of 1914. In Antiterra and Uchronic Paris a “tremendous cannon” of that name already operates in 1870, an intentional uchronic anachronism: the time distortion of the shared universe lets war technology arrive early.
The entry describes it both as the uchronic equivalent of the historical weapon, brought forward, and as the same Big Berta displaced in time, not a different cannon with the same name.