Military post offices on the Salolica front. French Aviation Mission with the Serbian army. Registered cover fr. with 35c. (registered letter rate to France) with French Unit marking "ESCADRILLE M.F.99.*LE COMMANDANT" and posted from "TRESOR ET POSTES*504*30.9.16", arr. "ST.NICOLAS DU PORT*9.10.16". The 1st cover seen from 1 of these units with postages paid, as required for reg. letters.