POSTES AUX ARMEES T.O.E. 16.8.1947 (=Territoires d’Opérations Extérieures / Territories of External Operations. It indicates service or mail linked to French military operations outside metropolitan France) on F.M. (Franchise Militaire: Free military mail "On active Service") cover addressed to ΑΘΗΝΑΙ ΑΦΙΞΙΣ 14.IX.49 (Athens). The letter was forwarded earlier to Paris, where a 25F postage stamp was applied (free franking by the Base Army Post Office), tied with POSTE AUX ARMEES 222 27.8.1949, RR military mail to Greece & VF

BPM 409 in 1949 belonged to the CEFEO military postal service in Indochina. It was one of the fixed Bureau Postal Militaire offices that served French forces in Vietnam.
Information drawn from philatelic studies on the Poste aux Armées:
• BPM 409 operated as a CEFEO postal hub in Saigon.
• It handled units with S.P. numbers in the South Indochina area.
• Its postmarks appear often on soldier mail from Cochinchina between 1947 and 1952.

• S.P. numbers in the 560xx range in 1949 are documented as linked to CEFEO rifle units in the Cochinchina – Nam Bộ zone.
• The 8e RTM was in the Saigon – Mỹ Tho – Vĩnh Long area during the summer of 1949.
• Mail from 8e RTM units was routed through BPM 409, which matches your cover.
• Philatelic collections already show S.P. 5604x strikes with confirmed links to the 8e RTM.
So for 16-8-1949:
S.P. 56044 = a unit of the 8e Régiment de Tirailleurs Marocains in the Saigon area.