The EAA91 is a double signal diode. The AA indicates two diodes with separate cathodes. The Philips/Mullard naming convention also uses B to indicate two diodes in a single envelope. The EAA91 or the EB91 are the same valve electrically but the EAA91 is physically smaller by 10 mm as the datasheets confirm.
The twin diode sections with electrostatic screen between.
The face view of one diode. The active area is small with the rest of the anode acting as a heat sink.
The other diode and screen. Note how clear the glass is.
The screen is embossed for rigidity.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 37 mm tall.
References: Datasheet & 1040. Type EAA91 was first introduced in 1946.