It is a stowage box for the M42. That said, remember that the M42 was given to the BW in numbers in the 50s and 60s. The specimen vehicle pictured with the questioned box shows several other items of German army modifications including blackout lights, taillights, the jerrycan, etc. My guess is that the German made box was added in the modification process. The box content could serve as something as mundane as an aux. fire extinguisher or such. Inspect the box closely and tell us what markings you find on it. It may, minimally include a 12 digit NSN number (NATO stock number) 0000-00-000-0000 and the makers mark and/or name.
