Ki-21 Sally remote-control tail "stinger"?

[Johnny .45]

Hey all. I've been looking into the Ki-21 since I'm hopefully receiving a die-cast version of it in the mail soon (although it's been three weeks since they said they were shipping it, so I wonder...). One thing I can't figure out for the life of me, even with the excellent cutaway drawing I found elsewhere on this site, is how the tail-armament worked. It's a "stinger" type 7.7mm machine gun, mounted in the extreme tail of the plane. There is no gunner. I've seen FIXED installations like this before (typically Luftwaffe), but this is specifically called "remote-control" in most of the references I've seen of it. Remotely controlled by whom? There is no crew member with a direct sight line to the rear, unless perhaps the dorsal gunner, and the view would be terrible. What do they use to control the gun? Somehow an electronic-type remote control system such as the B-29 had seems unlikely for a mass-produced early war Japanese type, even without a complex firing computer to compensate for range and deflection. Mechanical linkages are possible, but also seem implausible. And none of this explains who controls the weapon, regardless of how it's controlled. Without a remote gunsight and controls directly linked to the weapon, there would be no way to tell where it's aiming except the tracers.
Could this be a mistake, and it's actually a fixed defensive gun meant mostly to scare fighters away from the dead-astern position and into the field-of-fire of one of the other weapons? Or at best, maybe it's one that can be adjusted to fire higher or lower, but not really aimed per se? No matter how I think about it, it is not making any sense. Just wondered if anyone here had any thoughts or theories on this. Or the facts would be welcome, if you have them!