Negative, the positions are more toward the side if you look carefully whereas the one on the j31 picture is closer to the spine.. Besides the opening is hexagon in shape which would make it unlikely to be for the gun in addition to the position.
I agree that it is unlikely to be the gun position (I doubt they would show that detail on a model anyway).
I was just saying that your generalization about guns not being "on top" is also incorrect, or at best, it is vague.
Many planes have cannons that are not easily reached simply by crew on the ground, including F-22 and F-35.