
This cover has several aspects that set it apart from typical ARVN training covers of the period.
It was sent by a soldier training at the Thu Duc Infantry School (KBC 4100) in 1972 on a piece of United Nations stationery. He was in the 5th class of 1972 (Khoa 5/72). I am guessing that TBTX is an abbreviation for "Tham Bao Thien Xa" or Recon Unit Sharpshooter. Tell me if you think I am completely off the mark.
Despite the western (or at least American) style lines in the upper left corner for a return address, the sender created a triangle in the lower left corner typical of Vietnamese military correspondence.
The stamp, a coil version of the King Quang Trung stamp (Scott 411a) pays a 6d rate from a military personnel to a civilian address. A Quan Buu machine postmark cancels the stamp.

The sender wanted to make sure the return correspondence was properly addressed, so inside he drew up what the envelope should look like, including the addition of 10d postage.
Based on this cover, postage rates in August 1972 were 6d from military personnel and 10d sent to military personnel. The 10d rate is common, but I don't recall seeing a 6d rate. I need to dig through my covers to see if I have others I have forgotten about. Or of course the sender might have just gotten away with using a cheaper stamp.