The X81 is a triode hexode mixer for use in communications receivers. The triode oscillator is designed to produce 10 Volts pk-pk to drive the hexode mixer. The all glass envelope is protected by an all encompassing aluminium screening can that also serves to hold the spigot carrying base in place.
The plain reverse of the aluminium screening can.
The base pins and spigot. Note the reduced thickness central section, this appears to be specific to M-OV.
The wide glass tube envelope is 33 mm in diameter, and excluding the B8B base pins is 60 mm tall.
Reference: 1040. Type X81 was first introduced in 1947. See also1947 adverts.