Lindsay Graham wrote:
There's also a small box headed "For use at the Admiralty" which usually has initials and dates in it. What does "Admiralty" mean in this context?
Within a decade of Federation, Australia and Britain arranged that the RAN would serve under the British Admiralty during time of war. This occurred in both world wars.
So Admiralty in this context is the department/ministry that administers the Royal Navy and integrated forces, e.g. the RAN, Norwegian navy ships and crews that escaped to Britain after the German invasion.
Note that while "serving under the Admiralty" certainly covers administrative matters such as professional evaluation, as per your example, it doesn't necessarily mean operational control. In the Mediterranean and Atlantic RAN ships were under Admiralty operational control. From mid-March 1943 RAN ships in the SWPA usually operated as part of the US Seventh Fleet.
And my guess - purely a guess - at "I.K." is Incomplete Knowledge, which is why there is no rating number for the category.