For a primer on Douglas drop tanks, see http://tailspintopics.blogspot.com/2011/07/douglas-low-drag-external-fuel-tanks.html
Now take a close look at this picture:
Normally, both the A4D and the AD would have 300-gallon drop tanks under the wings. In this case, however, the A4D is carrying 150-gallon tanks and the AD, which is in the standard tanker configuration, 400-gallon tanks.
For more on the AD tanker configuration, see http://thanlont.blogspot.com/2013/10/texaco.html