I don't think that's for water cooling, a single pipe does not allow circulation, unless there was a coil/loop of smaller pipe inside a sleeve.
My theory is the anode is an emittor/beam former, either for RF or possibly even x-rays. A grounded cooling jacket was likely connected to the ring, the HV negative connected by screw terminal to the sideways stud on the base, and a ring clip connected to the anode for HV +. The bottom of that cavity would be a transparent window for RF/x-ray (yes, there are end firing x-ray tubes, but I have only found diagrams not photos, but basically they seem to use reflected/deflected electrons). Certain x-ray tubes also have odd appendages like that with other electrodes inside (not sure what for).