The E91AA is a miniature all glass premium quality pair of signal diodes. Siemens have used the Philips/Mullard name convention for special quality valves by putting the numbers before the section type. Rather than using B for a double diode the code is AA for two separate diodes.
The design was for detector service in wide band equipment. This would typically be RADAR. The premium version is designed to be used in areas of shock and vibration such as aircraft equipment. The UK government procurement code is CV4007.
The domestic version of these diodes was the EAA91 and here the use was television video detector. The two diodes are completely separate almost flat structures separated by a flat metal screen.
The ends of the two diodes with the electrostatic screen in the centre.
The valve was Made in Germany. Is it possible that this was made after re-unification?
The thin glass tube envelope is 18 mm in diameter, and excluding the B7G base pins is 38 mm tall.
Reference: Datasheet. Type E91AA was first introduced in 1960.