my father was A/4/77th ARA in 1970-71........as you may or may not know, ARA used the cobra quite a bit differently than the line aviation battalions and the cav. The most notable physical difference being that the ARA birds almost always have 19 shot pods on all four wing stations. From what I understand, and all the photos and such that I have seen, it is very rare indeed to find a cobra in the "heavy hog" configuration unless it was ARA. My father has told me that the ARA guys did not like the 20mm equiped helicopters for various reasons, and I have reference photos of some of their 20mm capable birds being used exclusively with the 4 x 19 shot pods. I also think I have, or could get some photos of some of their birds with the "double thumper" twin 40 mm chin pods.......although my memory is fuzzy compairing what he has in his photo album, versus what I have in digital format.
Another main difference between ARA vs line aviation battalions or cav was that the ARA was tied into the divarty net, as I understand it.......and worked thru those channels, with 2 birds on hot alert ready to provide aerial fire missions at all times.........he has related to me that they flew quite a bit at night, providint fire mission support to units in contact, and that they hated to fly at night in the mountains of I corps.........in fact, the A/4/77th lost two cobras and crews from his platoon in a nightime midair. This link shows all the fatalities of cobra's in vietnam.....
http://www.armyaircrews.com/cobra_nam.html
He also has told me that the ARA would fly the red portion of early and late pink teams with the 2/17th cav in order to suplement that units 9 cobra order of battle. (an ARA battery had 12 cobras)......
if you start with this link, and dig thru the various ARA, CAV, and Line Aviation Battalions or AWC's there is a wealth of information on this website...
http://www.vhpamuseum.org/defaultmenu.shtml