An "M3" style letter sheet with a Registry label is shown front and back. The cover was sent on 15 Nov 1972 franked with 24 dong in stamps - in addition to the military free-frank stamp printed on the letter sheet.
The registry rate was 15d through November 1972. How to account for the additional 9d in postage? In this case, the free-frank stamp apparently did not cover the basic letter rate. The military letter rate was 5d and the civilian rate was 10d. The domestic air mail rate was 18d. Any ideas how the 9d was calculated - or was the amount paid simply incorrect?
The second cover, sent express or for "urgent" delivery was sent from the Vietnamese 8th Marine Battalion at KBC 6618 on 9 August 1973. The cover is franked with 50 dong in postage, covering 10d for the civilian letter rate and 40d for express service.
Thanks to John Carroll for the updated rate tables.