Here is an interesting link. Search for "dorsal" on the page.
http://www.researcheratlarge.com/Air...Summary13.html
This is a report from 1943 "Notes on Japanese Aircraft". In particular it describes a cashed "Sally" and some of its features. Here is the section on the guns carried.
"The armament consisted of;
1 x 7.7 mm free gun in the nose,
2 x 7.7 mm free lateral guns just below and forward of the leading edge of the tail plane.
1 x 7.7 mm ventral gun under the leading edge of the tail plane
1 x 7.7 mm free gun in the tail, operated by remote control from the dorsal turret.
1 x 12.7 mm free gun in a dorsal turret."
All sights are ring and bead sights
except the tail gun which has the reflector type.
The cone of fire from the tail gun is about 20 degrees. A report on one crash states the possibility of the aircraft being equipped with a power gun turret."
Not quite sure how to interpret the reflector "sight" "on" the tail gun except it say it just was not used used in the air. With the shroud on the tail gun any sight on the tail gun would be worthless. It was probably used to sight the tail gun and align it with the dorsal gun when the plane was on the ground and the shroud was off the tail gun. Since it was "linked" to the dorsal gun it would need to be aligned and "sighted in" and I suspect that was the purpose of the reflector sight on the tail gun.