The PenDD4020 is a Mazda second-generation double-diode-pentode. The PenDD4020 had rather better gain than the Pen453DD and was correspondingly more successful, being used in a good number of AC/DC Murphy sets from around 1936 onwards. A good valve but, interestingly, its eventual end of life was often due to failure of the diode cathode emission rather than weakness in the pentode section.
With side light the lettering is a little clearer.
The top mica. The anode is pressed sheet and all three wire grids can be seen. The diode anodes are incorporated above the bottom mica on the common cathode.
The classic envelope is 54 mm in diameter and, excluding the B7 base pins, is 126 mm tall.
References: Data-sheet & 1043. Type PenDD4020 was first introduced in 1935.