I am doing a little research to build a early F-4J Phantom II of VMFA-232 at Chu Lai Vietnam circa 1969. I found a photo on the "House of Phantom" website which shows F-4J 155811 with a centerline gun pod (bottom photo). From the the slightly off head on view, I can't tell if the pod is the Colt Mk.4 HIPEG 20mm pod developed for the Navy or the SUU-23 20mm rotary cannon pod developed for the USAF. From the head on angle, both pods look very similar, the SUU-23 having a greater diameter. I am leaning toward the Colt Mk.4 as it would have been USN certified for carriage on the F-4J.
K