The 7B7 is a high impedance variable μ amplifier pentode. It was designed for RF & IF amplifier use, with a tuned anode circuit. This exhibit has no Type designation visible but it came in a plain box with the Type printed on it.
The small gap at the base of the anode reveals the suppressor grid.
At the base the cathode, heater and grid supports can be seen.
The wide glass tube envelope is 29 mm in diameter and, excluding the B8B base pins, is 57 mm tall.