The 6BH6 is a high impedance, medium gain signal amplifier. It has a low heater current requirement and was designed for use in car radio and other mobile equipment.
The reverse is overprinted with the UK Government procurement specification CV3908.
Through the gaps in the anode all three grids can be seen. The control grid is just visible near the lower mica.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 46 mm tall.
References: Data-sheet, & 1040. Type 6BH6 was first introduced in 1947. See also1947 adverts.