Hi Tigbrand,
A very knowledgeable member on this site has kindly sent me a scan of the "Handbook for the 2-Pr Q.F. Gun on Mark III Mounting", dated June 1916, in which this mounting is described as a "High Angle Pedestel Mounting". The Handbook goes on to say "...The base plate is hinged to the superstructure at one side and on the other are provided two hinged screws, the nuts of which are fitted with handles. These screws clamp the base plate when in firing position to two lugs rivited to the superstructure. To house the mounting the screws are slackened one turn and swung into recesses provided in the base plate. The mounting can then be lowered into the well in the superstructure."
Drawings in the handbook show the mount folded down, so it would apear that this gun is for submarines, as you say. It is manually operated (not automatic).
Very little in the handbook re ammunition; all I could see was a small box on one of the diagrams, wherein it gave the following details:-
Charge (Cordite Mk.1) 3ozs 124gns
Projectile 2 lbs
Cubic inches in chamber 10.2
Cubic inches per pound 49.7
Density .558
Total cubic inches in bore 114.4 (I think, can't read it clearly!)
Volumes of expansion 20.45
Muzzle velocity 2000 fs
Muzzle energy 55.5 ft
The barrel was slightly shorter than that of the Mk II gun, but the charge wt given is the same as for the usual ammunition, so still no nearer knowing why is Falcon's case stamped for use with this gun only!
Roger