What is this remarkable piece on the background ?

During the French Indochina war, in the areas under Hô Chi Minh rule the inflation rate is skyrocking (basic postal rate for letters : 10d in 1950, 1000d early 1953). Paper raw material being scarce, the service stamps to be used on administrative mail, are denominated in kilogramme(s) of rice, basic source of the vietnamese food , the price of which following the inflation rate. Four denominations have been issued : 0k600 basic rate for letters, 1kg, 2kg and 5kg.