I assume they like birthday cake and can have two birthdays, like the Queen does, as (apparently) they didn't become 'Royal' for another 5 months...
"The Australian Air Force was formed on 31st March 1921.
King George V approved the prefix "Royal" in June 1921 and became effective on 31st August 1921.
The RAAF then became the second Royal air arm to be formed in the British Commonwealth, following the British Royal Air Force. When formed the RAAF had more aircraft than personnel, with 21 officers and 128 other ranks and 153 aircraft..."