The SP-18 Red is a dull emitter AF amplifier valve from 1926-1928 and has a thoriated tungsten filament. Metro-Vick using the Cosmos trade name introduced a number of short-path valves in the 1920s. The valve carries the later style of BBC stamp and thus we can date this example to about 1927.
At the base of the composition cap moulding the diameter is 30 mm.
The essential construction employed in the short-path valves was a flattened tube anode and a grid attached to a single stout support. The filament is a single strand threaded through the centre of the grid.
The anode is clearly seen in this view.
See alsoSP-18 Red for another example of this valve type.