Thanks for that link Bertrand, but I think the information is giving me a headache!
And I thought the Japanese style of numbering groups was confusing. The French seem to follow the same pattern.
Ok lets see if I'm on the right track. Lets take the 3rd EC as an example.
3rd EC is a Groupement de Chasse (GC) - a fighter wing similar to, but not as large as, a German Jagdgeschwader (JG). In 1939 it was made up of two Groupes - GC I/3 and GC II/3. Each Groupe consisted of two Escadrilles (Squadrons) - generic No's 1 and 2 allocated to GC I/3 and No's 3 and 4 allocated to GC II/3. Again the Escadrille numbering is similar to that of the German system ie 1 & 2 always with GC I/... and 3 & 4 with GCII/...
Now if the above is correct, where I get lost is that each of those Escadrille's 1 & 2, 3 & 4 etc, had their own identifying original number. For example Escadrille 1 of GC I/3 was SPA 88, Escadrille 2 of GC I/3 was SPA 69, Escadrille 3 of GC II/3 was SPA 37 and Escadrille 4 of GC II/3 was SPA 81. Now unless those Escadrille are only ever allocated to GC I/3 or II/3 how would anyone know what was the actual Escadrille allocated toa Groupe?