I would guess that it was for replenishing hydraulic fluid. We used something similar in our DC-7s although ours used a hand pump and a 5 gallon can of hydraulic fluid. Skydrol for our ex-United Airlines DC-7s and 5606 for our ex-Eastern Airlines DC-7B.
The 5 gallon can in the foreground is Skydrol in Tanker 66.
2587C42B-35AE-4989-9CE3-408B55A4E845 by
tanker622001, on Flickr
My initial thought was a foam injection tank, but we used a 30 gallon plastic drum for that. We only used foam as a short term retardant for a brief period in the 1980s or 90s. I forget which. The idea was to take off with a load of water in the retardant tank and when we were sure that a drop was needed we would inject the foam concentrate into each compartment of the retardant tank. It took about 10-15 minutes for the foam to mix properly in the water.
The reason for using foam was that long term fire retardant was getting ridiculously expensive and if were canceled because the drop was no longer required we could just jettison the water. Even if the foam was already mixed it was a fraction of the cost of retardant. A 3000 gallon load of long term retardant cost about $15,000. The foam concentrate was maybe a couple of hundred dollars at most.
The disadvantages of using foam were that it was next to useless in heavy fuels or for making retardant lines unless fire crews were immediately available to follow up on the drop. A more serious problem was that the foam scoured the inside of the retardant tank and caused serious corrosion issues and loosening seals. Also, if you dropped a load of foam and then tried to load long term retardant it was difficult to fill the retardant tank because the retardant tended to foam like a poorly poured glass of beer.
I only remember using foam a couple of times on grass fires before it was decided that it was not a good idea to use foam in large air tankers for the reasons mentioned above.