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 directly into the gap in the anode along the grid axis a bright plate can be seen. Thus this is a beam tetrode rather than a true pentode.
The triode on the left is separated from the pentode section by a bright screening plate.
The thin glass tube envelope is 20 mm in diameter, and excluding the B9A base pins is 48 mm tall.