I don't remember where I got that "fact" but it seemed plausible, since I've seen a lot of pictures of Vietnam deployed carrier aircraft and don't think I've seen the gun pod but once on an A-4 (http://www.skyhawk.org/5e/g151022/html/151176c.htm) although there are also reports of it being loaded from time to time on Phantoms. (http://www.tomcat-sunset.org/forums/index.php?topic=2816.45;wap2) However, I did read that the OV-10 crowd used it with greater frequency and enthusiasm. The pictures are probably from Hughes testing circa 1958.
I really don't want to get into a contentious discussion, and I'm over 40 years past any sense of needing to defend Hughes or the Mk4, but your "facts" are really skewed. What I'd like to do is set the record straight. The Mk4's story is an interesting one, a story that began in Nazi Germany and led to Vietnam, and I hate to see it slip through the cracks.
The A4 squadrons out of Lemore, at the time the Navy's primary A4 base, were heavy users of the Mk4. Sure there were reliability issues early on, but those were less of a problem after 1968. How much the gun was used off of the carrier is a complex question. First of all, the gun pod was most useful in close air support (CAS) missions. The ordnance loads typically flown off of the carrier were bombing missions or air-to-air missions. The gun pod wouldn't be that useful--remember an A4 also had two Mk12 20mm as internal guns--so the Mk4 would have been sitting on the c/l station where a MER could hold multiple bombs. It's also interesting to note that in the article you cite, the author says, "the Mk-4 gun pod seemed prone to jams, especially under high G's (F8's had same problem with their guns)." What he's taking about, and it's noteworthy that the F8's guns were the old reliable Mk12s, is a load being carried for air-to-air missions.
The Marines, on the other hand, flew many many CAS missions, so they were heavy users of the Mk4. A typical CAS load for an F4--both MAG11 and MAG13 were F4 groups--was a Mk4 on the centerline rack with napalm and/or CS on the outboard racks. The technique doesn't sound very nice from the perspective of 40 years, but at the time it was simple: drop the nape or CS on a suspected pocket of NVA or VC, then as soon as they started running go in with the gunpod. I spent a year and a half with the Marines in I Corps of RVN, and except for a few weeks when we were tracing through problems with ammo--the fuzes were coming off of HEI rounds and FODDING the radome of the F4s--we saw lots and lots of usage; we're talking hundreds of thousands of rounds fired. And while I wasn't working with MAG12, the A4 group in Chulai, the A4s would frequently carry multiple--usually two--gun pods, so their round count would be higher. I think a concise testimony to the Marines respect for the Mk4 was seen in the Spring of 1968, when one airplane belonging to VMFA-122 was rigged to carry and fire three Mk4s.
The use of the Mk4 on OV10s, however, was pretty much restricted to one squadron, VAL-4. The Black Ponies were working in the Mekong Delta and used the gun pod on their centerline station. Bear in mind that VAL-4 was land based, operating from Binh Thuy. Yes, they were enthusiastic, but VAL-4 was a real anomaly in many ways, so it's hard to equate their mission or attitude to any other Navy squadron. I worked with VAL-4 for a short time and flew one combat mission with them. They were hot-shots in every positive meaning of the expression, and I have tremendous respect for them and the way they approached their mission.
As for the photo, it could well be from 1958. What couldn't be seen in the earlier photo--the one I'm sure is from China Lake--are two important clues: the first is the doors of the pod. Production doors had louvres that were about 10 inches long, probably about 15 of them. The louvres ran perpendicular to the centerline of the pod. The ones in the photo are smaller. Second, and even more telling, is the--my memory of exact nomenclature is fading--barrel locking mechanism. If you look closely at the photo, you'll see the two barrels running to the gun, then they terminate at a very rectangular block. That rectangular block is a locking mechanism that both locked the barrels in place and worked as a cylinder for gases being vented from the "first-fire" barrel. The first-fire barrel (in the photo it's on the top, as the gun was loaded into the pod upside down) was vented and operated as a piston to cycle the gun. By the time the gun went into production, that very rectangular block had been reshaped and was more like a figure eight than a rectangle.
