This is an amalgam post. So not a copy of a particular unit, person or battle.
A popular method of carrying more ammo or larger mags was to use canteen covers as pouches. They can hold a lot more magazines that the standard M1956 and are tall enough to accommodate 30 rounders as well. Additionally the ARVN ruck was occasionally seen in use.
FYI ARVN rucks would be more common with USMC (last pic is later war marine recon) units as the Army started started issuing the lightweight and tropical rucksacks pretty quickly. Doesn’t mean soldiers didn’t use them (particularly earlier on) but it would be less common than those. The CIDG or NVA backpacks are also seen.