Weapons on a Venom

I'm painting up my first Venom and the model seems to have 3 weapon arrays. There's one cast into the underside of the hull, which looks splinter cannon-esque. There's a second which glues (magnetizes) to the underside which has two options of a cannon or twin linked rifle, and then a 3rd splinter cannon mounted on top with a gunner.
The venom only has two guns. Either the gun built into the underside of the hull is something else, like an air intake, or the top mounted gun isn't necessary. Anyone advise which it is?

The 2 weapons are the cannon on top and the mounted single cannon or twin rifle underneath. The other thing on the bottom is a sensor array of some kind and has no in game effect