There may be a simple timeline and explanation for the introduction of British-style field service caps in the AIF written down somewhere, but until someone comes up with it (please!) below is some information on hats/caps at Gallipoli (from the AWM), which may, or may not, be of help. I was going to speculate on what all this meant re exactly when the caps were first issued, but have now thrown the towel, rather than my hat, in the ring.....
It seems there was an attempt to get a handle on the hat situation on Gallipoli in June, and in response to a request for info the following reports were generated:
Divisional HQ, Anzac Cove, 15 June 1915, to the OC 3rd Brigade: "It is noted that there are 1705 serviceable hats now in possession of your unit, made up as follows: -- 9th Battalion none at base, 102 with unit; 10th Battalion -- 164 at base, 21 with unit; 11th Battalion -- 291 at base, 102 with unit; 12th Battalion -- 900 at base, 125 with unit.
Please state, for the information of the General Officer Commanding, the reason for the great shortages shown in the cases of the 9th, 10th, and 11th Battalions. In regard to the numbers given for the 12th Battalion, I am to convey to you, for information to the CO, the GOC's appreciation of the care taken to preserve the hats of that Battalion, and the consequent saving of expense."
On the 16 June the 1st Australian Division sent the following to Anzac HQ: "Many of the units of the Division left a number of their slouch hats at the base at Alexandria. The GOC of the division would be glad if instructions could be sent to the base for these to be extracted from kitbags etc, sorted and packed by units, even down to platoons and troops, if practicable, the packages carefully marked as to contents, and unit, and despatched to Anzac. The GOC further desires to request that the officer in charge of the kits of the Division at Alexandria, be instructed to ensure that the overhauling of kits for the purpose of obtaining the hats be done under the personal supervision of responsible non-commissioned officers. This is most desirable as, unless attention is drawn to the necessity for it, the interests of the owners of the kitbags as regards the safeguarding of the contents thereof, may not be sufficiently conserved."
In an undated document, obviously generated in response to the June 'hat memo', the 1st Division, including attached 4th Light Horse and 2nd Light Horse Brigade, reported 1849 hats left behind at base, and 2604 "serviceable hats with unit." The 1st and 2nd Brigades reported virtually zero hats left in Alexandria, the four battalions of the 3rd Brigade reported over 1300 hats left behind.
The only concrete evidence I have to date of issue is from the 3rd Field Ambulance, which was issued their caps after the unit embarked for Lemnos -- here is Pte Gunn's diary for the 4th of March, 1915:
"Pretty smooth and had a good night's sleep. In the morning we had our caps issued out. The are the same as the Tommies wear and are rotten, unhealthy things. They are only given to us to protect the officers."
Although Gunn was a cynic, there was a general feeling of antipathy among the men toward the officers of the 3rd Field Ambulance at this stage, so it is possible the 'protecting the officers' theme was common.
The 4th Brigade Medical officers sent a report to NZA Division HQ on 28th of June stating: "In this weather the ordinary Field service cap is insufficient protection for the back of the head and neck, and that helmets, felt hats, cap flaps, or some other form of protection is advisable." Col Monash added: "If these were obtained, they think the habit of wearing little or nothing on the body could be continued without prejudice to the men's health. In view of the above I would be glad to be informed as to when cap curtains or felt hats are likely to be available for issue to the Brigade under my command."
In the last week of November 1915 the 13th Battalion reported that 287 men possessed caps only, 163 hats only, and 121 had both hats and caps. The CO also considered "that any hats at present in wear on becoming unserviceable should be replaced by hats, being the more suitable head dress for winter." The 16th Battalion reported 462 men in possession of caps, 166 with hats, and 69 with both. "The general feeling of the Battalion is preference for hats," wrote the CO.
2nd Australian Division HQ replying to Anzac HQ on 23 January 1916 stated: "The most suitable type of headdress depends upon the time which the Army Corps may expect to spend in Egypt. If it is probable that we will leave Egypt by about the end of April or early in May, the hat is the more suitable. It is more suitable for wear then the helmet in any climactic conditions other than summer in Egypt or the tropics. It is also the distinctive Australian head dress and is preferred on that account. If the Corps is to spend the summer in Egypt the helmet will be more suitable."
I agree with Hendo that the slouch hat seems ubiquitous in the AIF after 1915, but only overseas. I have photos of 2 relatives, taken in 1916 and 1917 in Perth before embarkation, in which both are wearing the field service cap. Conversely, I have dozens of photographs of 3rd Field Ambulance individuals and sections, taken in camp in Australia, on troopships, and in Egypt, in 1914 and pre-March 1915, and all OR's are wearing slouch hats, or (usually in camp in Aust) the 'garrison caps' modelled below by Ted Langoulant.
As for the photos taken on the May 24 armistice posted by Peter, the second one was taken by Pte Kyle Gault of the 3rd Field Ambulance. The first one is very similar in view to another photo taken by Gault that day, which appears in Vol.12 of the Official History. The 3rd Field Ambulance was evacuating the RAP's of the 3rd and 4th Battalions at the head of Bridges Road at this stage, and on May 24 one squad from each section and 50 stretchers were sent up to help the 1st Brigade burial parties.
Good on you,