Bonjour
First , according to that site ,the B2551 was a Sopwith 1/2 strutter
http://www.ukserials.com/
When coming in England the 4 AFC became 71 squadron RFC intil 20th january 1918 when it became again N° 4 Squadron AFC
The 'british aviation squadron markings of WW1' book (Les Rogers) gives the following indications:
"Squadron Marking
Allowed by 23rd Decembre (1917) ,a white boomerang behind the cockade and repeated on the top decking.A new squadron marking was allotted 22nd March 1918;this was a single white bar in front of the cockade
Individual markings
"A" flight used letters A,B,C,D,E in front of the cockade
"B" flight used numbers 1,2,3,4,5,6 in front of the cockade
"C" flight used letters U,V,W,X,Y,Z in front of the cockade
In march 1918 the establisment was raised to 24 aircraft."A"flight added "G" and "H","B" added "7"and "8",and "C" flight added "S" and "T".All were repeated on the upper starboard wing
After the new squadron marking was allotted on 22 march 1918
"A" flight used letters A,B,C,D,E,F,G,H behind the cockade
"B" flight used numbers 1,2,3,4,5,6,7,8 behind the cockade
"C" flight used letters U,V,W,X,Y,Z,Y,Z behind the cockade
All markings were repeated on upper starboard wing"
In september 1918 the squadron was re-equipped with sopwith snipe
Cordialement
Bruno
A photo of the Camel B7406 of 4 AFC can be seen in the book among others
here, with serials numbers allocatd to 4th AFC
http://www.adf-gallery.com.au/afc14.htm
and here is the profile of the B7406