This cover was sent by a Vietnamese Ranger (Biet Dong Quan) via registered mail from KBC 4926. This was the Duc My Ranger Training Center in Nha Trang. The other number is the soldier's military number or So Quan. The stamps are cancelled with a military Quan Buu postmark 1 October 1968.
The most interesting aspect of this cover is the label affixed at the bottom. It was applied in transit by the (non-military) post office. The text reads, "the parcel was damaged when received," and "the cover has been repaired."
The label is unusual in that it contains both Vietnamese and French text at a date when French was no longer an official language. By 1968 French was taught in school as foreign language but most school kids chose English for practicality. Perhaps there was a stockpile of these labels from the 1950s.