HMS Southampton "Y" turret being lowered into place 3 July 1936
Photograph copyright by Upper Clyde Shipbuilders / Ian Buxton

Bow Turrets of HMS Apollo (later HMAS Hobart) in February 1938
U.S. Naval Historical Center Photograph # NH 58634

The same turrets on HMAS Hobart following torpedo damage in July 1943
U.S. Naval Historical Center Photograph # NH 80533

A Turret being fitted to HMS Bermuda in May 1942
Note the roller paths
Photograph copyright by Upper Clyde Shipbuilders / Ian Buxton

Breech of 6" (15.2 cm) gun on HMS Belfast

Engraving reads as follows:

B.L. 6 In Steel Mark XXIII
R.G.F.
N. N^oS/4580

Where:
B.L. 6 In Steel Mark XXIII = Breech Loading 6 inch (15.2 cm) gun of All-Steel construction Mark XXIII
R.G.F. = Built at the Royal Gun Factory
Note:  A manufacturing year should follow "R.G.F." but I cannot make it out
N. N^oS/4580 = Navy Gun Serial Number 4580

Note that the projectile is stenciled with "EMPTY"
Photograph copyrighted by Vladimir Yakubov

Illustration showing the differences between BL and QF guns
Click on this sketch for a larger image
Sketch from "The Gunnery Pocket Book, B.R. 224/45 (1945)"
Used here by permission of Historic Naval Ships Association

Click on this sketch for a larger image
Sketch from "The Gunnery Pocket Book, B.R. 224/45 (1945)"
Used here by permission of Historic Naval Ships Association

Cut-away drawing of 6"/50 (15.2 cm) Mark XXIII Mounting
This sketch does not show the set-back of the center gun
Sketch from "The Gunnery Pocket Book, B.R. 224/45 (1945)"
Used here by permission of Historic Naval Ships Association