This exhibit is a mystery. The only identification, apart from the maker, is either the 123 or the BR. The construction is as a cold cathode trigger tube. Raytheon use BR plus numbers for similar types of valves, so it is possible that this a BR and the first in the series.
The open cylinder is the anode. The projection at the top is just the getter holder and connects to the anode.
Inside the open anode cylinder is a central cylinder and a rod within. The cylinder is probably the cathode and the central rod the trigger.
The balloon envelope is 38 mm in diameter, and excluding the UX4 base pins is 75 mm tall.
Updated July 11, 2014.