Type EL821 is a video output amplifier used in professional applications for driving TV or radar signals down (fairly) long cables. We do not think that it was much used, if at all, in domestic sets.
Sources specifically list it as a video amplifier. Note the high value of gm, necessary in order to obtain reasonable gain and power output with wide-band signals, and for correct matching of video into coax cable. TV studio equipment racks used to be full of 6CH6s.
The valve is pin compatible with the 6BW6 but they have different uses.
The anode is mainly closed and prevents a view of the internal structure.
