In early WW II, some U.S. Navy carrier-based torpedo and scout bombers were equipped with the ASB radar. These were identifiable by the presence of a Yagi array antennas under the left and right outboard wings.
These were individually aimed by the radar operator in directions of interest and the return could be interpreted for navigation (identification of harbors, coastal cities, and ships) and attack at night and from above the clouds. Ironically, the antenna was invented by a Japanese scientist before the war.
It took some training and skill to operate the ASB radar. It was replaced by the APS-4, which looks more like a conventional radar. It could be mounted on an existing bomb rack and jettisoned if needs be. Airplanes wired for the APS-4 received an "E" suffix indicating modified electronics. It is sometimes mistaken for a fuel tank or a bomb. (For more on the APS-4, see http://tailspintopics.blogspot.com/2014/11/things-under-wings-anaps-4.html
The APS-4 was replaced by the APS-19, a more capable radar, for most new applications beginning in 1946. (The TBM-3S conversions used for the interim ASW hunter/killer team were still equipped with the APS-4, however.) When provisions for the self-contained radar pod were standard, as in the Douglas AD-4, no E suffix was required. In general, when the APS-19 was installed in night fighters it was an integral part of the aircraft rather than carried as a pod. The Grumman F8F was an exception.
The APS-31 appears to have come in two different flavors from the standpoint of antenna size. The APS-31 or -31A carried by the Grumman AF-2S pod was similar to the APS-19 pod except that it had a longer and more pointed tail cone.
The APS-31B pod that was hard mounted (albeit removable) under the right wing of the AD-4N and AD-5N was much larger than the APS-31 or -31A on the AF-2S. The pylon was unique to and integral with the APS-31B radar and unlike the one it replaced, perpendicular to the wing surface instead of perpendicular to the ground.
The AD-5Q could utilize both an APS-31C and an APS-19:
Rick Morgan noted that the airplane pictured above might have been taken when VAW-13 was involved in Operation Water
trying to intercept North Vietnamese aircraft dropping supplies in the
south since the APS-19 had an air-to-air intercept mode.
The following illustration is based on drawings that aren't up to my usual standards and pictures. Better information would be appreciated. e.g., it appears that there may have been an AD-4N pod radome that wasn't quite as "blunt"as shown here.