Oooh, so close!
It's a Cashuat, a vehicle produced in El Salvador in the 80s (still in service) in conjunction with the US Army's Tank Automotive Command. The vehicle was indeed in series production, but was based on the old US Army M37B1 truck. The US designed the vehicle itself, cut the armour and then shipped it for assembly in El Salvador... Apparently, there're still 40 or so kicking around that part of the world.
I'll admit I'm impressed, FatwagCpl...