Okay, I found my instruction sheet and am trying to remember exactly how that part fixes to the fuselage half. J111 has to be glued on to the thick vertical frame at the front of part E28. It is the instrument panel for the gunner/radioman and if you have a picture of the rear cockpit looking forward you can see it on the left side. Sorry I'm not more descriptive, here's a photo of my old build for reference:

Douglas SBD Dauntless 1/48 Eduard
J111 is the small white instrument under the decking on the left side. It's almost impossible to see when the gun ring is installed!