The X76M is a frequency changer designed for series heater chains. The X76 looks to be identical as both have an outer metal paint coating.
With the zinc paint flaking off the inner bright anode can be seen.
The cathode is a round tube and the control grid is wound on copper supports. The control grid is connected to the top cap. The two screening grids are connected together at the top. The triode oscillator is mounted below the mixer and cannot be seen.
In this side view the screening grids connecting strap can be clearly seen.
The wide glass tube envelope is 28 mm in diameter, and excluding the IO base pins is 86 mm tall.
References: 1040 & 1043. Type X76M was first introduced in 1946. See also1946 adverts.