The abbreviation "M.N." on this postmark stands for Mien Nam, as the Communist regime referred to the "southern region" or liberated areas of South Vietnam.
The use of red ink is unusual, but the main appeal to me is its use in conjunction with a military stamp. Mien Nam postmarks typically canceled Mien Nam stamps, such as the ones depicting Ho Chi Minh watering a tree. Military stamps and M.N. postmarks are not often found together.
Tuy Phuoc is near Quy Nhon, and near the site of Champa towers built centuries ago.
UPDATE: This cover is a fake. For whatever reason I was only paying attention to the postmark and the fact that it was applied to a military stamp, but I didn't recognize the obvious - that the stamp was issued in 1982. Thanks to Ta Phi Long and John Carroll for noticing and calling it to my attention.