The BD-400 is a beam switching tube from Burroughs and used for pulse counting. The device need an external magnet and internally electric fields modulate the electron beam from terminal to terminal. The outer metal cylinder is not magnetic. So far no operating details have been found for this device. Later products have integral magnets e.g. the Mullard ET51.
The maker's name.
The top of the cylinder is closed with a plastic cap.
The glass envelope is flared at the base to accommodate the 26 pin base.
Looking directly at the base pins. The filling between the glass and the outer cylinder looks to be filled with cork.
The outer metal tube is 45 mm in diameter and, excluding the base pins, is 73 mm tall.
References: Observation and above article and website.