Here is a 1990 Cambodian domestic letter sent from Mongkol Borei (spelt "Mongkol Borey" on postmark) of Banteay Meanchey Province to the national capital, Phnom Penh.
Mongkol Borei is the southernmost and most densely populated district of Banteay Meanchey. The district is named after a river of the same name which runs from Prachinburi of Thailand to Tonlé Sap (Great Lake) of Cambodia.
Mongkol Borei used to be in the historic province of Battambang. In the18th century Siam annexed northwest Cambodia, since then the area was under Bangkok's rule until 1907 when Siam was obligated to cede the conquered Cambodian territory to France under the Franco-Siamese Treaty.
In 1988 Battambang Province was split. Five northern districts including Mongkol Borei were separated to form the new Banteay Meanchey Province. Banteay Meanchey means the fort of victory, and Mongkol Borei means the blessed town. This land of victory and blessing is predominantly rural, important economic activities include farming and fishing.
Map showing Mongkol Borei with Banteay Meanchey Province in red.