Ordnance Board Memo B 6,412 of August 1923 makes the first mention of a proposed .8 inch automatic gun for aircraft observers and gives a specification. By March 1924 the Board was discussing a .9 inch autmatic gun, both these designs being by Elswick Ordnance Co, part of Vickers.
Later the same year Memo B 8,049 is headed “Aircraft Armament . Tracers, shot, 0.8 inch and 1 inch. design DD/L/1509” and talks of modifications to the shell driving band and asks Superintendent of Design to make 10 steel cartrdige cases to Design DD/L/1447.
The following Memo, B 8.050 talks about .8 inch and 1 inch “Aiming rifles” for the QF 4.7 inch gun. However, this appears not to be an aiming rifle in the sense of a training device but more of a dual calibre gun as TonyW describes. It states “It is thought the gun will seldom be used against armour plate, unless perhaps agianst tanks which would be a very special use. The weight of armour necesary to keep out a 0.8 inch or 1 inch bullet would probably be prohibitive in aircraft.”
It is unclear if the 1 inch cases are the same in both memos, but it seems likely.
I found no further mention of the 1 inch gun and the 0.9 inch gun was disposed of in 1925 although work on the .8 inch Aiming Rifle continued. However, Vickers may well have continued work on the 1 inch as a private venture.
One interesting comment is a note from Elswick to the Ordnance Committee that appears in Memo B 8,720 of April 1925 “As you are aware we have now adopted manufacture of the Colt automatic gun which it appears to us will fulfill the requirements of an automatic gun firing 0.9 inch ammunition.” I though at first this might be the .50 Browning (or Vickers-Armstrong Colt as they called it), but the O.C. replied “…I have no knowledge of the 0.9 inch Colt gun”.
P.S> Yes, Tony, “R.C.” is “rifle calibre”.