This exhibit is a mystery. It is clearly a screened grid valve, or tetrode, and the ratings we know of come from the lettering on the external metallising zinc paint. The style of screw-cap for the top connection placed the date of manufacture to between around 1929 to 1934.
The reverse of the envelope is plain zinc paint.
The Bakelite moulded top cap and screw terminal.
The balloon envelope is 45 mm in diameter, and excluding the B4 base pins is 107 mm tall.
