The W61 is a post war variable mu RF pentode for broadcast receivers. The grid bias range is given as -3.0 to -25.0 Volts.
The identification label is missing but would have been Osram or Marconi the two consumer brands of M-OV.
Little of the internal structure is visible from below. Looking into the gap at the top one can see the widely spaced turns of the suppressor grid that is located out towards the anode. Also visible are the two inner grids that are much closer wound and pass flat in front of the cathode.
The wide glass tube envelope is 29 mm in diameter, and excluding the IO base pins is 96 mm tall.