That system is the later Hughes-designed "black hole" IR suppression, which was also sold under the name "Ocarina".
The Vietnam OH-6s had field mods instead - there were several different designs done as field expedients. They all basically consisted of a metal cone to fit over the hind end of the engine compartment, and the exhausts were vented out through cuts in each side, above the engine clamshell doors. The design of the cover over the exhausts differed - I've seen photos of round ones, photos of squarish one, etc. Some appeared to be made of fiberglass or light metal, with some sort of sealant applied to the fuselage join. I've examined several OH-6As that had the field mods and then were de-modded back in the US, and all of the side-holes were different shapes, angles, etc. indicating no standardization.
At the same time that field units were making mods Hughes was also experimenting with an early IR suppression kit for the OH-6. There are small photos of it in a 1972 AWST article. It consisted of a tail cone and two largish "toilet bowls" which redirected the exhaust plumes upwards, not towards the back like the later "black hole" system. The standard black hole system as used on the IAF MD500s mixes ambient air with the exhaust plume. I don't know that Hughes ever fielded a standard kit in Vietnam - I've never seen photos of anything which is like the later standard "black hole" system.
I did see the black hole system fitted to an OH-6B in 1989/90. Several OH-6As of varying National Guard units were upgraded by the NGB in an effort to get the OH-6A in as a contender for the AHIP program. These few airframes were known as "OH-6Bs" and they had the black hole mod. The tail cone was held on by two metal struts which were welded to the interior of the upper engine area.
As far as photos of the Vietnam-era field mods, they are as rare as hen's teeth.
Two Hughes 500Cs (369HS) flown by Air America in 1972/3 also had side exhausts.
John Hairell ( snipped-for-privacy@yahoo.com)