Most airplanes of its days had a Dihedral, or a wing thickness tapering with a 'V' shape, this one is the opposite, hints exist as done to avoid: 'Dutch Roll', or that the ordinary way will impair performance. What is it??
This plane is very interesting as it highlights a few design options. Firstly, in seems to have neither significant dihedral nor anhedral, but it does have landing gear underneath it, lowering its CG for stability. Being low aspect ratio, the builder probably decided to forego dihedral as cross wind gusts could make for a rolly, uncomfortable ride. A slight sweep in the leading edges of the wing would provide some "dihedral effect" when rudder was applied.
Now for the wing itself, not albatross or eagle, we'll go flying squirrel here. But notice how low the landing speed is. This is a virtue of lower AR wings, though not as efficient as high AR (albatross) wings, they do stall at significantly lower speeds. This design was also applied to the Navy "flying flapjack" and it performed well.
And thanks for the article, the ads made good reading too!
I don't think it's a deliberate anhedral. It's just a geometrical consequence of having an airfoil with flatter bottom. If you imagine a plane connecting the leading edge and the trailing edge, it will be flat. Or likewise, if the airfoil had a completely flat bottom, the lower surface would be flat, and the top one sloping down.