A quick stab.....inter war years and the puttees are the real clue as they were worn up to 1937. I would think that the change came when the RAF modernised its structure for the future...the establishing of the various Commands....Bomber,Fighter and Coastal etc from 1936. The uniform shown is the blue service dress of an airman.At that time and into the war, the only issue was the dress uniform.The working uniform came later and was known as the "working blue"and the dress uniform was known as the "best blue" The badge on the airman's left sleeve is that of a gunner and the airman is likely to have been a AC1 or even a AC2 Many many years ago I worked with a senior engineer had been in the RAF before the war.Any discussion on service,he would laughingly retort , "Get some time in. I was in when we wore puttees"....it was a uniform legacy from the RFC.