The 15D2 is a heptode frequency changer. The CV1107 specification was based on the 15D2. The base cap also has the old stores code of 10E/383.
The 13 Volt heater was designed for series heater chains - I wonder if it was also used on vehicle 12 Volt vehicle batteries.
The 15D2 with an octagon border on the glass suggests that possibly STC used some manufacturing equipment from the parent ITT in America as the base says Made in England.
Looking at the line of grid supports.
Here above the pinch the wire grids and the oxide coated cathode can clearly be seen. The U shaped rod is the oscillator anode and will be just the two plain rods.
A closer view: the control grid first wire can be glimpsed.
The classic envelope is 38 mm in diameter, and excluding the B7 base pins is 97 mm tall.
