The UK Navy NGT5 (aka CV1144) is a directly heated hot cathode mercury vapour thyratron. The prototype was the BTH BT19.
The additional lettering is W1244 and C13093.
The base end reveals little about the construction.
Although difficult to see due to reflections from the glass, one electrode can be seen entering the at the base.
The top cap leads to a glass sheathed connection. The glass rod forms a fixing for the anode and the central anode connection is welded to the anode at the top of the anode disc.
The classic envelope is 54 mm in diameter, and excluding the B4 base pins is 137 mm tall.