[SAS1]

Its a shame I didnt get this a day earlier, as I was in the Gieves archives on Friday!!

Looking at the badge its not Royal Naval Air Service and is clearly not Royal Navy, so its certainly not pre 1918. The FAA was the air arm of the Royal Navy, but after the First World War effectively all came under the RAF, and it was only later that the RN took responsibility and all RN flying came under the Admiralty. Throughout the 20s and 30s it seems that both RAF and RN officers were seconded for flying duties with the RN, with particular insignia to denote this. Its certainly not an RAF officer attached to the RN/FAA as mentioned above, the denoting insignia for that was a small gold wreath in bullion with a silver anchor centre, worn on the lower left cuff of the RAF uniform, above the rank, abolished about 1938. FAA and RN history isnt my strong point, but looking at the elements:

The peak of the cap isnt particularly small, so I think that rules out a WW1 era cap. The Gieves label is around the 1920s era but exactly when and to it was used is hard to say. The Royal Warrants - Gives and Hawkes (as they are now) have three the label on the cap indicates two (the one on the right is Prince of Wales the other might be the King or the Royal Household).

As for the badge, it is not RNAS, but RN with the eagle in place of the anchor denoting an Air Branch Officer. I think I have only seen one before and again would indicate the early 1920s to late 1930s. Its impossible to pin down more fully the dates as there is no indication when the officer might have worn it. For example, in the Gieves archive they have a WW2 RN battledress but with a 'crownless' wing, which was only worn 1924 to 1938. That indicates an officer serving in the FAA (or RN) during the war, but wearing the pre 1938 wing he was originally issued despite it being obsolete.

The cap and badge look genuine and original so I would guess its a cap for a Royal Navy Officer attached to the Fleet Air Arm anywhere during the 1920s or possibly into the mid to late 1930s.