When I saw this first, I went to the famous 1 squadron photos showing
lots of aircraft.
This one and the ones like it.
I counted and counted but couldn't be sure they were extra, spares or whatever.
Then I came across the following in D Mintern's History of 73 squadron.
"Wind, rain and fog then put a stop to almost all flying until
mid-May [previous mentioned date is 25th April].
An order had been promulgated that all Camel squadrons would have an increase in establishment to 27 pilots and 25 aircraft, and this resulted in six new pilots arriving - Lts Leyson, Douglass and Mortimer, all USAS seconded to the RAF, and Hayes, Doane and Drewbrook the last an old member who had been struck off strength to hospital in mid-March."
So it looks like there was indeed such an order. It might have begun in March and only taken effect by mid-May.
Where the new pilots added to flights or was a new D flight created? I can't say for certain, but previously changes in flight commanders have been recorded, and there's nothing here about a new one, so I would say the new pilots were added to the existing 3 flights.