This tube was originally designed for low frequency amplification in WW1 communications.
The Telefunken name and numbers are etched into the glass, although somewhat faintly.
The black metal base has various areas of wear.
The bottom wafer is loose but holds firm, upon removal the opposite side is stamped "Germany" - see photo 9.
It is being sold as untested and for display purposes only since the working condition is unknown.
Other examples of this rare tube can be found in various museums across the world including the National Museum of American History. There is also a detailed write-up about this tube in the book 'Saga of the Vacuum Tube' by Gerald F.J. Tyne.
