There were three versions of this weapon.  The first was used on the Victoria Louise class and was constructed of two layers of hoops and a cylindro-prismatic breech wedge.  The second was for twin turrets used on the later armored cruisers and had an extra layer of hoops on the rear of the gun.  The third version was similar to the second but was designed for casemate mountings and did not have the extra layer of hoops.

Eight guns removed from the Victoria Louise class were used on the Belgian coast in the Freya and Hertha batteries (four guns each).  In May 1917, one gun removed from Roon was emplaced at Cape Helles, Turkey, where it was used to harass the British ships seen at Kephalo and Aliki.

These guns were used in coastal artillery units during World War II.

  Actual bore diameter was 20.93 cm (8.24").

Armored Cruiser Victoria Louise in 1909
Note mixed secondaries of 15 cm and 8.8 cm guns

Armored Cruiser Hertha

Armored Cruiser Scharnhorst about 1907
21 cm guns in turrets and in upper casemates
U.S. Naval Historical Center Photograph # NH 46837

Stern Turret on Scharnhorst
Note the built-up nature of the gun barrels
Photograph courtesy of The First World War