The 6D2 is the Mazda equivalent of the Mullard EB91, and consists of two separate signal diodes in the envelope.
The diodes were designed for vision and sound detection in television receivers. Additionally they are recommended for limiting and DC restoration.
The two diodes seen end on with the screen between them.
One of the diodes seen face on. The small central bulge is the working part of the anode, the rest is for heat dissipation.
This thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 43 mm tall.
References: Datasheet & 1040. Type 6D2 was first introduced in 1953.