This RW3 travelling wave tube is new and has serial number 3757. It is rated for service at 4.2 GHz and to deliver 5 Watts of RF power. The gain of this tube is 37.8 dB. The two critical voltages are 1340 and 625 for g2.
In operation the tube would housed to provide a magnet lined cavity and waveguide input and output ports.
In this second picture the electron gun can be seen. The cathode current would be 38 mA
Something that is not seen with integrated TWT Amplifiers is the helix, that critical guide for the slow wave.
The fixing ring at the base is 52 mm diameter.
The thin glass tube envelope is 18 mm in diameter, and excluding the base pins, is 285 mm tall.
References: Data-sheet & Tag with valve. Type RW3 was first introduced in 1962.