The ECF804 is a triode pentode designed for application in wide-band amplifiers and instrumentation. Wide band amplifiers require a large mutual conductance, and thus initial high gain, to allow feedback to broaden the bandwidth and flatten the pass-band, all at the expense of gain.
Looking into the anode the pentode can be seen to be a beam tetrode. The beam forming plate is the bright structure with the grid supports visible through the gap in the centre.
Between the triode on the left and the pentode on the right is the electrostatic screen.
The thin glass tube envelope is 21 mm in diameter, and excluding the B9A base pins is 48 mm tall.