M-OV had trouble designing the MPT4. Not only did it take them a clear two years to produce technically successful copies of Mazda's landmark AC/Pen but thereafter they had to redesign it several times (including the beam tetrode version MKT4) over the next 5 or 6 years.
The interesting point about the grids is their shape.
The the suppressor grid is close to the anode. The inner grids are held on supports away from the cathode but are shaped to form a small cage around the cathode and close in.
The balloon envelope is 56 mm in diameter and, excluding the B5 base pins, it is 117 mm tall.
References: Data-sheet & 1043. Type MPT4 was first introduced in 1931. See also1931 adverts.