Does anyone know model of these army pants with just 1 back pocket or something similar to them?
The basic type of pants is referred to here as HBT's - short for "Herringbone Twill" - the type of material they are made from.
Looks to me that there is a seam and maybe a small hole on the left side where the pocket used to be that slants downward from upper left to lower right.
Could be that the other pocket got torn off.