1964 pattern Lightweight Nylon Rucksack (attached to its frame), issued to US Army soldiers (also worn by Marines to some extent) during the Vietnam War.
seen here stacked with bandoleers for 556mm magazines and 40mm grenades, canteens, and an M43 Folding Shovel
Photorealistic snap button texture by Justin N.
Base rucksack model by Jujurat
example references:
https://www.worthpoint.com/worthopedia/vietnam-era-1964-lightweight-rucksack-1848431345
https://www.worthpoint.com/worthopedia/ultra-p64-experimental-lightweight-1834727270
https://cdn.discordapp.com/attachments/662820879717105687/749399201460256868/image0.jpg