The 7B8 is a heptode or Pentagrid converter. 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.
The outer wire grids have a narrow pitch.
The identification is etched into the top of the envelope.
The wide glass tube envelope is 29 mm in diameter, and excluding the B8B base pins is 56 mm tall.
References: Data-sheet & 1043. Type 7B8 was first introduced in 1939. See also1939 adverts.