John.
I don't think it is military at all but a home made shooting seat for the back of an open backed truck,used to chase and kill game from the back of a vehicle..
Such as this one shown on this site...
Scroll down to the Seeland shooting seat..The seat belt indicates mobile application,rather than static application.
http://www.euroguns.co.uk/acatalog/Shooting_Sticks.html