The EB91 is a double signal diode. These diodes were designed for vision and sound detection. Additionally they are recommended for limiting and DC restoration.
This is an import as the valve is labelled Foreign. Mazda not wishing to identify the country of origin.
Both anodes are dark grey on their outer surfaces. This improves radiation efficiency for the heat generated but is unusual in such low power signal diodes. The EB91 anode is normally made from bright nickel sheet.
ace view of one anode. The cathode tube has been formed with a bulge to prevent it from sliding through the lower mica. The insulated heater wires are clear in this image.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 37 mm tall.
References: Data-sheet & 1040. Type EB91 was first introduced in 1949. See also1949 adverts.