God only knows what that is! Lister water tanks were large steel drums, somewhat larger in diameter than a 45 gallon oil barrel and about the same height, with fittings top & bottom. No lids IIRC, but I could be wrong there. Again IIRC, the drain-off taps were fitted at the engine end, rather than the barrel (the engine being the lowest point of a thermosiphon system).
This looks original, although it may not be:
As you can see, the fuel tank is strapped to the water tank, I believe this was a fairly common setup, can't verify it though as the only Lister I've ever seen in its original location was an air-cooled VA...