Hey guys the @The-Old-CJ IS RIGHT!
the bag is there Under Gunpouches section of the .rar mod.Thank U very much mate for providing the link.

Also there is another great texture of this backpack in here:
https://www.gta5-mods.com/player/vietnam-grunt-trevor-outfit

@toitoine66 has done another great job in making the texture of the bag like being used in the old war days.
Thank U all.