The TH4B is a triode hexode frequency changer. The triode acts as the local oscillator and the hexode as the mixer. For maximum isolation of the input signal from the oscillator and output circuit the input signal is taken to the control grid via the top cap.
The base has split pins and the lead out wires are soldered into them.
Seen from the top, where the metalising paint does not extend, only the top cap and within the top mica is clearly visible.
The wide glass tube envelope is 35 mm in diameter and, excluding the B7 base pins, is 128 mm tall.
References: 1040 & 1043. Type TH4B was first introduced in 1938.