"Have Gun Will Travel" according to the decal sheet above was UH-1B 64-14040. She served with 2/20th ARA C battery (That's what the red circle on the door signifies) from October of 66 to December of 67. More info on her HERE. Just like my dad's bird, she went on to serve as a Seawolf in HAL-3 in 69 and 70. The M21 gun system first appeared in Vietnam in 1966 as an experimental setup with the 173rd AHC. So I'd say it is theoretically possible 14040 could have carried the M21 system while with the unit but the M16 system with the quad M60C's and M157 or M158 7 shot rocket launchers was probably more likely.
Regarding the seats, I have done quite a bit of research on them and while I wasn't there I can tell you that many if not most armored seats in country were OD green with black trim. There were certainly gray ones as well. Here are some pics that will show what I mean. First batch are from Mike models that I have taken. Second set are in country photos like those posted by Gene above. Third set is at the Ozark Huey II facility where each seat gets rebuilt and re installed. As a side note, all UH-1H seats were painted black sometime in the 80's when Hueys got NVG compatible cockpits.
Ray
Pics by me of Museum aircraft (Note that the red handle is held in place by a cotter pin. when removed the red lever can be flipped down to allow the seat to tilt backwards aiding in removing an injured pilot or co-pilot:
The seat above is missing the armored panel on the inside.
Here are some Vietnam pics that show OD seats:
Bob Coveney 190th AHC:
Crewman with last name Alexander of the 129th AHC
Wrecked Huey:
John Logan 190th AHC:
Not sure of the unit in this one:
Here's a set from Robert Brackenhoff of a 174th AHC seat that might be gray. Honestly, i can't tell for sure.
Finally, here are some shots of the refurbed seats that will go into new Huey II's. All recent armored seats are black.