I was with the 173rd Airborne. Our original mission was to protect Ton Son Nhut airbase but that was soon expanded to include War Zone D, the delta and in other areas in the sothern portion of Viet Nam.
Some regulation were too stupid to obey. I had a round in my chamber but they would check, usually when they fly hot meals in, someone would be checking weapons. Easy enough to get around.
I don’t know anyone that got in trouble for having a round in the chamber so that tells me the regulation came from someone way away fromt he field.
I have to admit I never heard the term War Zone D. I rode Vietnamese boats in the Delta in Three Corp and Four Corp as an advisor.