The SS215P from Six Sixty is an audio output triode that is equivalent to the Mullard PM2. Six Sixty at this time were retailing Mullard seconds.
The electrodes are obscured by the blackening of the envelope, this is an azide process valve. Looking up from the base the anode box becomes visible. The supports show that the flat horizontal structure consists of a single V filament and the grid.
This valve was an ebay find and was sold as unknown. The markings are difficult to see and are formed in the glass envelope by etching with hydroflouric acid. It took many attempts to photograph the markings with any success.
Part of the exercise to reveal the detail was to reduce the image to a negative.
The same lighting allowed the other markings to be seen.
The balloon envelope is 45 mm in diameter, and excluding the B4 base pins is 92 mm tall.
Reference: 1043. Type SS215P was first introduced in 1928.