The square box with the pipe setting between the exhaust pipes is a heater for preheating the engine during cold weather. The crew would dig a trench and place the heater in the bottom. After getting a fire going much like a charcoal grill fire, they would drive the tank into the trench and over the heater. Then the whole thing would be covered with a tarp for the night. Interesting solution to cold starts in sub-zero weather.
On my last one, I added a .015 shim to the top of the tail gear leg and added the assembly after everything else was complete. This improved the sit of the model and eliminated breading off the tail wheel during construction. HTH, Dave.