I am indebted to Pierre Pepin of Philatelie et Aviation Militaire for discovering this cover and sending it to me.
The cover, postmarked 29 Nov 1953, is struck with a nice Secteur Postal / Le Vaguemestre hand stamp. The digits at the bottom matches the sender's return address of SP 4103. The style of this hand stamp is a bit different from others I have seen, so I was happy to add it to my collection.
As I've noted before, French postal zones utilized five-digit Secteur Postal numbers to obscure a unit’s designation and location. Under American advisement, the Vietnamese adopted a four-digit Khu Buu Chinh (KBC) code system that served the same purpose. During the transitional period of 1952-1956, one finds various combinations of French and Vietnamese text on the postal markings and addresses of military covers.
I was able to locate the listing for KBC 4103 in the tome Danh-Luc Buu-Khu (1970) and identify it as serving the 2/43 Infantry Battalion headquartered Long Khanh within Long Nai Province in the southeastern section of Vietnam. While there's no guarantee that SP 4103 was in the same location in 1953, I'd say the chances are pretty good in was in generally the same area.
As always, if anyone has has more information, please add a comment to this post or email me directly.