The basic AD-5 retained the belly dive brake of the three that were on single-seat Skyraiders, retracting into a well in the belly the same way.
Note how the bottom of the fuselage at the trailing edge of the wing curved into the well.
Since the AD-5W did not need a dive brake, I had assumed that its belly would have had a panel covering the well (the AD-5Q does). It turns out that the dive brake was removed but the well itself was now the bottom of the fuselage.
Ed Barthelmes, my go-to guy for Skyraider stuff, sent me pictures showing this area and maintenance manual information.
Note that there is an access door in the forward end of the well. This was standard on the AD-5 for "kit conversion and (access to the) electronic equipment compartment". Note also the dark rectangle. This was the cavity to accommodate the retracted dive brake actuator since provisions were retained on the AD-5W for installation of the dive brake.
http://tailspintopics.blogspot.com/2014/02/the-widebody-skyraider-redux.html (this one has a side-view drawing showing the changing depth of the well)