The 1258 is a miniature triode Thyratron designed for use as a modulator in small radar systems. The circuit in the attached PDF shows the basics. Operation is rated to 10,000 feet but with arc-over protection in the base it can be used to 80,000 feet.
The peak anode current is 20 Amps and the pulse repetition rate is up to 5,000 pps.
The cathode is to the left and anode at the perforated end of the outer screen (connected to cathode).
The anode end of the screen.
The cathode end.
The thin glass tube envelope is 21 mm in diameter, and excluding the base pins is 44 mm tall.