The 6BW7 is an internally screened pentode. It is the Brimar implementation of the standard valve, the Mullard EF80 is the more generally known type designation. While it was designed to occupy the same function zone it is not a straight replacement. It is, however, pin compatible.
As with the EF80 the circuit designer is given maximum flexibility by having all electrodes brought to external pins. Many pentodes have g3, the suppressor grid, internally connected to the cathode.
The thin glass tube envelope is 20 mm diameter and, excluding the B9A base pins, is 47 mm tall.
References: Datasheet & 1040 Type 6BW7 was first introduced in 1952.