After placing that first point, you can shoot backwards, unlike the Hellhound for IG. At any rate, the entry for the Night Scythe does say the Tesla Destructor is turret mounted. Granted it is in the fluff description. So that does not directly relate to rules, unfotunately. However, the Death Ray hangs below this with one, ball-shaped connection. I'd be willing to listen to arguments for either type of mounting. But I'm pretty sure the consensus would fall to hull-mounted, especially being mounted on a Flyer.
"Fetch me another plaything. This one seems to have broken." -Urien Rakarth
Space Marine- "This planet is ours, witch."
Eldar- "No ... This planet is theirs ..."