This is a cover which I believe most casual collectors would simply throw away. Only one stamp on it, it is a definitive - boring and most of all, damaged.
Look closer before you send it to landfill. The postmark has made things unusual.
It is a "PHNOM PENH RP CAMBODGE" postmark, all in French. "RP" is the acronym of "Recette Principale", the main post office of a city or an area, in here refers to the Phnom Penh Central Post Office. "COA" is "Courrier Ordinaire Arrivée". This "COA" postmark is meant for incoming ordinary mail, it is an arrival postmark.
Now you are enlightened. This cover is an outgoing mail, "COA" should not have been on it. "COD", "Courrier Ordinaire Depart" is the right postmark for outgoing ordinary mail.
The reason for this mistake is unknown. So far I have only seen one more sample of such mistake, it is a stampless official cover from the Department of Posts.