The 7B8 is a heptode or Pentagrid converter. This exhibit is an E variant but no specific details have yet been found.
The Pentagrid is a self oscillating mixer with the triode oscillator being formed between the common cathode and grid two. In many such valves the second grid is formed from just the two support rods.
The loctal series can normally be matched to similar valves on octal bases. In this case the appropriate valve to compare it with is the 6A8.
This valve looked to be new old stock but when opened it was found that the glass was separated from the base at the level of the metal skirt.
The method securing the grid wire to the support is revealed. The support is notched along the length and where a wire has been placed into a notch the metal has been squeezed back to hold it. This is called notch and peen.
The screening grids are joined at the top.
In this image the cathode tube can be seen to be coated only where the electron emission is required.
The Type designation is 7B8E the milky internal coating was once the getter eliminating the final traces of gas.
The internal coating was most likely present to stop stray electrons from liberating gas from the glass.
The wide glass tube envelope is 29 mm in diameter, and excluding the B8B base pins is 53 mm tall.
References: Data-sheet & 1043. Type 7B8E was first introduced in 1939. See also1939 adverts.