This exhibit is a classic early Type R made for the British Government (note the 'arrow' marking). It is probably of late WWI vintage. We expect that this particular sample was made by GEC-Osram before their immediate post-war reorganisation as a result of which they became M-OV.
The cylindrical anode is threaded by a helical grid and within that passes the single strand of Tungsten wire as the filament. In use the filament would burn brightly just like a light bulb.
The spherical envelope is 52 mm in diameter and, excluding the B4 base pins, is 89 mm tall.
References: 1005. Type R Type was first introduced in 1916.