In my article, Vietnam 1955: Military Postal History in a Year of Transition, in the March 2008 issue of the Indo-china Philatelist, I referenced numeric QUAN BUU markings 1,2,4,5 and 8, in addition to the number '7' marking that was the primary subject of the article.
Shown is a tan registered mailing receipt from a sender at KBC 4218 within I Corps.
The pink form appears to be a declaration of value for the contents of the letter. Of note is the QUAN BUU 1 postmark on the back of the form, dated 26 November 1956.
The abbreviation "CTTT" in the address on the pink form is spelled out in the address on the tan form as "Cong Thu va Tao Tac." This roughly translates as "Corps of Engineers". "Cong Thu" literally means "government buildings" and "Tao Tac" literally means "buildings and construction." (Thanks, Vinh for decoding these!)