The 425/Pen is a late 1920s battery output pentode with an output power of 750mW.
The B4 base with the side terminal for the screen grid. This side the gettering is thin and has a slight milky appearance. The other side is mirror bright.
The electrode supports are taken to a glass bead to the side of the anode and so this could be an Azide valve.
The dome carries the identification.
The three grids can easily be seen in this image.
In this view the two filament strands can be seen together with the dark metal supports.
The box anode.
The balloon envelope is 50 mm in diameter, and excluding the B4 base pins is 107 mm tall.
Reference: 1043. Type 425Pen was first introduced in 1929. See also1929 adverts.
