This exhibit is probably an M-OV type L610 low-impedance battery triode but types H610, HL610 and P610 also existed and looked fairly similar. Professional users, including the Marconi Company (MWTCo.) preferred to use 6V battery valves long after domestic users had switched to using 2 V types for economy.
Thus, although the M-OV '610' series date back to the later 1920s, the style of labelling on this specimen dates it to around 1938. The specimen looks new and was probably supplied as a spare for existing equipment, but never needed. These valves were well made and had long lives.
The electrode structure is hidden under the opaque glass envelope.
The balloon envelope is 44 mm in diameter and, excluding the B4 base pins, is 87 mm tall.
References: Private communication.
Updated November 03, 2016.