WIII
I saw a UH-1M with the INFANT system in late 69. I may be wrong but I think that all of the old C's were converted to M's later, and I think that the big difference was the replacement of the engine with the T53-L-13. The Army was using a bunch of old UH-1M's at White Sands Missile Range as target drones. A look at the FAA Type Certificate Data Base does not show any UH-1C's or M's. I still have my old AH-1G dash ten, as well as the ones for the AH-1S (and F's). I would rather be in a Cobra in a fight than a C or M because we carried more bang and were skinny, faster and hard to hit. Just don't do your breaks over the target.
Are you referring to War Zone D? I was stationed at Phu Loi (near War Zone D), then Soc Trang, and Can Tho.