The 0A4G is a late 1930s cold cathode thyratron from RCA. The data-sheet gives the range of uses that this gas relay was aimed at.
The glass tube with the metal rod inside is probably the main anode. The wide disc is the cathode and the trigger or grid will be within the central gap in the cathode.
If the disc is the cathode the grid is the short rod on the left of the central hole. The main anode is shielded by the central glass tube and this prevents early breakdown of the gas.
Thanks to Mark Mallison for sending the images.
The classic envelope is mm in diameter, and excluding the IO base pins is mm tall.
Reference: Data-sheet. Type 0A4G was first introduced in 1938. See also1938 adverts.