Here is the front and back of a cover offered by a Bulgarian dealer.
Look at the item description by the dealer:
"A cover sent from Kampuchea Cambodia to Bulgaria in September 1991 during the last days of the pro-Soviet and pro-Vietnamese RPK regime that had no recognition by UN but de facto ruled Cambodia between 1979 and 1991 as opposed to the Khmer Rouge Pol Pot parallel regime supported by China and USA. The civil war and the isolation of Cambodia form the Westerns World ended in October 1991 with Paris Peace Accords and the introduction of the United Nations Advanced Mission in Cambodia.
"Postmark from 30.09.91 with a variation of the RPK/People’s Republic of Kampuchea postmark “Phnom Penh RP Kampuchea” different from the other postmark formats used through the 1980’s and into the early 1990’s “Phnom Penh RP” only.
"Variation of stamps both with Kampuchea and Cambodia inscription (the name of "People's Republic of Kampuchea" was officially changed to State of Cambodia in April 1989)."
There are 3 things highlighted by the dealer. Firstly the cover is from the last days of non UN recognized PRK, secondly the postmark is unusual, thirdly there is mix franking of People's Republic of Kampuchea (PRK) and State of Cambodia (SOC) stamps.
The cover sounds so much like a gem! I have repeatedly reminded fellow collectors to stay alert of dealers' trap, this is one of the traps.
PRK renamed SOC in April 1989 merely for the sake of international image, no one considers it a political or social milestone.
The Paris Accord was signed in October 1991, but the date did not mark the end of a regime. It was 1993 when Cambodia re-installed a monarchy, so basically the postmark date 30th September 1991 gives absolutely no significance of any kind.
Not to mention mixing franking of PRK and SOC stamps can be found on most covers of the period.
The dealer takes time to read Graham Shaw's website and makes it a reference of the postmark, the postmark on the cover is the same type as shown in figure 5 of Shaw's article. Personally I do not consider numbered postmarks as a variation, and it is certain that this postmark type is not unusual, according to my study the numbered postmarks are extensively used from 1987 to 1993. For further information see my blog entry "Phnom Penh CPO Postmarks Part 1".
I guess only very junior collectors would consider having covers from non UN recognized countries a fantasy. There are quite a number of non UN recognized countries around, such as Republic of China (Taiwan), Turkish Republic of Northern Cyprus, Republic of Kosovo, State of Palestine .....
The dealer asks for 75 USD. The true market price, is just a few dollars.