The Pen36C audio output pentode is a B7 version of the CL4. The CL4 has a Ct8 base and the valve was also issued with an International Octal base as the CL33. The B7 base was issued in 1933 and this valve was designed in 1935 for use in AC/DC sets wit no mains transformer and a series heater chain.
Mullard tended to add the Suffix -C to a type fitted with a non-standard base cap for sale in a particular market and this may be true here. If so the valve originated with Philips in Holland.
The coating within the glass obscures the inner detail and this top mica reveals little/
The anode is carbonised for better heat radiation. The cathode tube has a reduced diameter at the top to fit into the top mica. The grids are all wire but there seems to be an extra set of vertical rods within the anode that appear to be the top mica supports.
The classic envelope is 44 mm in diameter and, excluding the B7 base pins, is 118 mm tall.
References: Datasheet & 1043. Type Pen36C was first introduced in 1935.