The XR1-6400 is a Xenon filled Thyratron with reliable operation over a wide temperature range. The envelope is also marked 5545 - the STC version Type designation. The Mullard advert referred to above describes the XR1-6400A with a disc-seal construction that would supersede the XR1-6400.
The design use is for speed control of DC motors. Also specified for counting and sorting devices and electronic switching machines.
The filament needs 60 seconds heating time. The Thyratron is a pulse device and the anode volts and current are limiting values the average current is limited to 6.4 Amps. When conducting the forward voltage drop is 16 Volts. The grid can go negative by a maximum of 250 volts. The deionisation time is 500 μ seconds.
The coating on the inside of the glass obscures the inner details but here is a glimpse of the electrodes.
The anode top cap connection. The stranded wire is enclosed in a glass tube so as to force the discharge to where the designers expect it.
The wide glass tube envelope is 60 mm in diameter, and excluding the 4 special base pins, is 190 mm tall.
Reference: Datasheet. Type XR1-6400 was first introduced in 1959.