The B36 is an audio double triode with a 12.6 Volt heater. It is a British (M-OV) copy of the American 12SN7. This exhibit has a clear envelope and the twin triodes can be seen. The B65 is identical electrically but has a 6.3 Volt heater.
The Marconi brand label has seen better days.
The heaters are bifilar wound to minimise the external AC field.
The cathode connection to the support steps away from the other supports.
The wide glass tube envelope is 29 mm in diameter and excluding the IO base pin length, stands 73 mm high.
References: Data-sheet & 1040. Type B36 was first introduced in 1948.