Here is an unusual item - a promotional flyer for baby formula from the "SMA" company. It was sent to Doctor Mai Le Hiep at the Cong Hoa hospital in 1965. This was the largest military hospital in Saigon, the other military hospital in the city was the operated by the Navy along the Saigon River.
The military hospital had its own KBC number (I'll try and find it), but the sender apparently didn't know the number so it was addressed directly to the hospital and placed in the civilian mail on April 3rd. A meter paid 0.80d postage. The cover was transfered to the military post office for delivery and received the military "Quan Buu" postmark on April 5th.