In April 1992 at the request of the UN Secretary General Boutros Boutros-Ghali, the People's Republic of China sent 47 military Observers and 400 engineers to aid the rehabilitation of Cambodia during the UNTAC time.
Engineering units from 5 countries participated in the rehabilitation mission: PR China, Thailand, Japan, Poland and France. PR China was the first time to join a UN peacekeeping mission and the brigade did not have their own field post. I have not come across any philatelic documentation on their mail, yet a 1994 book titled "Blue Helmets from China" by Zhengfeng Xu, a veteran from the mission, provides a very scarce peep at how the Chinese wrote home. Here is a rough translation on the relevant lines (original text in Chinese):
"... in the beginning it was difficult to send mail to China and very difficult to receive mail from China because of the mail route problem, there was no mail exchange between China and Cambodia.
"According to UNTAC, mail to China was to be handled by the Force Communications Unit run by the Australian contingent. However we had no PO Box in China ... and so things did not work. To solve the problem, the former Engineering Corps Department arranged ... to assign the Beijing Box 3947 at 46 Taiping Road for use, the postcode is 100857 ...
"... in August 1992 ... the UNTAC HQ assigned Air France to take Chinese mail twice a week, postage is paid by UNTAC. The mail route is Phnom Penh - HCM City - Bangkok - Beijing. Each week the Beijing International Post and Telecommunications Office picks up mail at the aiport from Air France, then through the 3947 box, the mail is delivered with military post cachet applied. And so the mail is by special delivery, it has no postage stamps, it only has the blue UNTAC datemark along with the red triangular (Chinese) military cachet.
"Now it is convenient to write to us, just send the mail to Beijing Box 3947 with 20-cent stamp franked would do ..."
The Chinese People's Liberation Army (PLA) was so proud of the Brigade that in 1993 when was the 66th anniversary of the founding of the PLA, China Post issued a commemorative cover (serial PFTN-1) to hail the Brigade for their contribution to the UNTAC peace mission. The print run was 10,000, an unknown amount of them were postally sent from Beijing to the two Chinese Engineers Brigade camps in Phnom Penh.
Those covers sent to Phnom Penh have the Beijing 1st August 1993 postmark, it was the date marking the 66th anniversary of the PLA foundation. The UNTAC 16th August postmark tells the arrival date.
A special handstamp cachet with a stylised pigeon and the inscription "In commemoration of the participation of Chinese Engineering Brigade in UN Peacekeeping Mission in Cambodia" was also issued for stamping.