The VMS4 variable μ screened tetrode dates from 1931. It was designed for RF and IF amplification in broadcast superhet receivers.
This exhibit has the screw top cap assembly. The screw top cap was phased out around 1933-4 and so this is an early version of the design.
The circular skirt near the base connects to the mesh screen grid.
The screw top cap. It looks as if the Bakelite covering the top nut has been lost.
The balloon envelope is 45 mm in diameter and, excluding the B5 base pins, is 125 mm tall.
References: Data-sheet & 1005 Type VMS4 was first introduced in 1931.