Great pics, thanks. They definately look like they are using foam, so I'd guess they used a similar foam / water mix with compressed air like the one used with the HH-43. That last pic looks like the guys have a hose reel in the cabin with them, or it might just be the winch cable.
I worked with a guy who was a firefighter on an HH-43 in Vietnam so I have a little understanding how those worked. It was pretty amazing the way he described it, the helo would drop off the FSK and the two firefighters, then use the downdraft of the rotors to push the fire away while they made a pathway to the pilot with the foam.
Compressed air foam units are starting to show up again in the fire service and they really are not much different than the FSK except they are smaller and use a different type of foam. They make nice lightweight, fairly maintenance free fire "pumps" on small vehicles like fire department utility pickups, chiefs trucks etc. Vehicles not intended to fight fire but are likely to find fires in their daily driving around so its nice to be able to do something with a smallish fire. I have a photo from the 1930s of a US Forest Service pickup with a similar device too. That one used dry ice to make CO2 which pressurized the water tank.
I was just curious how it worked since the tank on the side of your huey looked about the size of those wheeled fire extinguishers you see at a lot of helipads. I could see someone getting the idea of taking the wheels off one of those and bolting it to a skid.
I don't know anything about these outside of figuring out how they probably worked, but I'm a firefighter so I find these things reaaly neat, and I'd love to see any close ups you have.
You probably know this but in case you don't the FSK (Fire Suppression Kit) was a firefighting unit slung under the HH-43. It kind of looks like a giant golf ball and a hose reel on a sled with a small ladder and misc rescue tools attached to the side.