I have posted a couple of pictures regarding the rear wing fixing. These are from a pre-war car where there is a large hole to allow access to the suspension arm. As you may be able to see…the captive nut is within the “space” between the inner and outer sills. Post war cars do not have this access hole. I believe the only way to repair the captive nut is to cut open the sill, fix and make good!
Probably should have annotated them – bolt securing the wing is on the right side of the “hole” covered in crud.
(this is a 1939 Light 12…unrestored….!!!)