The People’s Republic of Kampuchea existed from 1979 to 1989 as a satellite state of Vietnam. It was founded in Cambodia by the Vietnamese-backed Kampuchean United Front for National Salvation, a group of Cambodian communists who were dissatisfied with the Khmer Rouge due to its oppressive rule and defected from it after the overthrow of Democratic Kampuchea, Pol Pot’s government. Brought about by an invasion from Vietnam, which routed the Khmer Rouge armies, it had Vietnam and the Soviet Union as its main allies, as can be seen from these stamps. (From Wikipedia)
People’s Republic of Kampuchea > Cambodia