To me it looks like most of G4M2 bombers had this tail.It's permanently open for some kind of Jap 50cal or 20mm cannon.
Tail gunner of G3M1
had this view
Excellent film with G3M1 and very slow walk around
https://www.youtube.com/watch?time_c...&v=x4xEaF7H1AA
More info in this post.Those guys do some good research imho:
https://forum.warthunder.com/index.p...-model-errors/
Lots of good drawings here
https://www.sas1946.com/main/index.php?topic=57252.0
