The Cosmos SP55 from Metrovick was introduced in the 1928. Thrower gives two variants SP55B and SP55R. The B for RF or RC and the R for audio. This exhibit is only marked SP55 and was possibly made before the production was divided into two types.
The SP or Short Path refers to the construction method that enabled the grid to be set closer to the filament than conventional cylindrical designs of the time.
The pip has been broken and the vacuum has been lost. It can be seen that the construction flattens the anode as much as possible.
The small front to back dimension is about 3 mm.
The base carries the MetroVick logo, Cosmos Brand, Shortpath and SP55. The P indicates the anode pin.
The control grid winding is secured to the support on one side only. Note the corrosion to the grid wire. The filament wire is oxide coated.
The balloon envelope is 50 mm in diameter and, excluding the B4 base pins, is 102 mm tall.
Reference: 1043. Type SP55 was first introduced in 1928.