Here is a cover sent in April 1966 from a member of a Vietnamese Recon company (dai doi tham Bao) operating from KBC 3338. Based on the enclosed letter, the sender is writing to a girl in Saigon whose address he got from a buddy in his unit. This is his first attempt to befriend her.
"Tien tuyen" in the upper right corner is a term used to indicate the sender is in the front line, and "hau phuong"means in the rear or city, so "tien tuyen goi hau phuong" literally means "front line to the rear."
Thanks to Vinh Nguyen for the translation.