This cover postmarked 7th October 1985 went from Phnom Penh to Cầu Ngang of Cửu Long Province, Viet Nam. It bears the "5th Kampuchean People's Revolutionary Party Congress" hand stamped propaganda cachet in red ink.
The period of use is unknown. A reasonable guess is two weeks to one month before the congress.
Closer look of the Khmer-French cachet. It carries the hammer and sickle - symbol of Communism, and five-towered Angkor Wat silhouette - symbol of the nation seen on the national flag of People's Republic of Kampuchea.
In 1985 the Kampuchean People's Revolutionary Party (French acronym "PRPK"), sole legal and ruling party of Marxist-Leninist Cambodia at the time, held its 5th Party Congress from 13th to 16th October.
The first 5 Year Plan of People's Republic of Kampuchea was presented by General Secretary Heng Samrin at the congress which was to call for an economic reform boosting private economy and closer tie with Viet Nam and the Soviet Bloc. The plan labeled agriculture and forestry as the real force of national economy with highest priority given to agricultural production focusing the 4 sectors of food, rubber, fishing, and timber.
PRPK changed its name to the Cambodian People's Party in 1991, the group remains the ruling party of Cambodia till these days.