Following my call for help of yesterday, I got a feedback from Ha Tram (Thanks a lot) who confirmed that the stamp from Cambodia I showed had been issued to commemorate the 40th anniversary of the diplomatic ties between Vietnam and Cambodia. She also sent me a scan of the FDC of the complete set. Here it is.
This is not a joint issue, since only Cambodia issued stamps to commemorate this event. The set was issued on the 14th of June.
I’m still looking for information about the sculptures that are pictures on the set of stamps.