You are correct sir! The bottom is solid balsa sheet. I was incorrect on my "not sheeted at all" comment. The structure is not stick built then sheeted, it is more of a soft balsa box with lightening holes cut into it on the top and sides.

Regardless, the structure needs beefing up in my opinion. How did yours fair over time? What engine were you using? My tail shook so bady that the elevator was a blur after one season of flying.