The Lissen AVC2 is a directly heated valve designed for battery sets. The pentode is of the variable μ RF type and this gives a clue to the use and the name. If the pentode is used as the IF amplifier the diode would be the AGC rectifier. AGC was originally known as AVC for Automatic Volume Control. The signal detection would be carried out elsewhere as diode signal detection was not the norm until later in the 1930's.
Lissen also produced an AC/AVC diode pentode that had a B5 base with the cathode connected to pin five. The heater was the UK standard of 4.0 Volts at 1 Amp.
We have no dimensions for these valves but taking measurements from the image gives the approximate values presented below. The data-sheet and image come from an ebay listing.
The top mica showing the three grid supports with the inner grid connected to the top cap. The filament tension spring is in the centre.
This photographic donation shoes the etched lettering much better than the actual exhibit.
The classic envelope is 49 mm in diameter, and excluding the B4 base pins is 120 mm tall.
Reference: Data-sheet & 1043. Type AVC2 was first introduced in 1933.