The Panels behind the rear gun are folded down, that was done to increase the area the rear gun could cover. I also did not know the panel on the base of the vertical fin folded down with the decking. I had to go back and look at picture #6 to compare the area, you can see the fillet lines in picture #6. It may be the angle or the lighting that makes picture #5 look odd. Below is a picture with the gun deployed and the area does not look so odd.
Source; Wikiwand - Curtiss SB2C Helldiver
My Father flew SB2C's and I built the old Monogram kit as a Kid, I should have known what rear panels folded down, if I did I forgot. The next picture is of a model, but it shows the base of the fin folded down.