This is the famous ARVN rucksack
"The ARVN rucksack was a small to medium sized pack with an external but close-fitting steel frame with "X" cross bars through the center. The pack had an open top with drawstring over which a flap closed, secured by web straps which ran down the length of the pack. A tab on the flap with eyelets allowed the attachment of any American M1910-style equipment with wire hangars, but most often the M1943 entrenching tool and cover.
Two rectangular, flapped pockets large enough for a Meal, Combat, Individual (C-Ration) could be found on either side of this attachment point on the main body of the pack. Webbing points and a pair of metal eyelets on either side of the pack and above each pocket allowed the attachment of items using wire hangars or slide keepers, which featured on items of American M1956 Load Bearing Equipment and M1967 Modernized Load-Carrying Equipment.
A pair of adjustable web straps on each side and on the bottom of the ruck allowed for the attachment of items such as bed rolls, shelters, and other cylindrical items like spare machine-gun barrel bags and M72 Light Anti-tank Weapons. Simple padded, adjustable shoulder straps and a waist pad allowed for comfortable carriage. The pack sat high on the back, not interfering with items placed at the rear of a combatant's equipment belt, unlike the low-riding American Lightweight Rucksack of 1965 and Tropical Rucksack of 1967."
ARVN rucksacks were never an official item of United States military equipment. Those which saw service were either acquired individually or by units through trade or purchase with members of the Army of the Republic of Vietnam. In the first few years of United States involvement in the Vietnam War the M1956 Field Pack (butt pack) proved too small to carry combat loads, but the Lightweight Rucksack and the M1951 Mountain Rucksack in the inventory at the time were in high demand and short supply, so many units and individuals acquired ARVN rucksacks for use in the field. Many were replaced in United States military use during the later years of the Vietnam War by the Tropical and Lightweight rucksack as more became available. Those examples which left Vietnam with US troops did so as souveneirs and did not see use outside of Southeast Asia.
This item comes with US markings (see inset picture). More pictures on demand. Some light signs of corrosion due to the rucksack age.