The M-OV X61 triode hexode was made in both clear and metallised envelope versions. This has a clear envelope but on the extreme left can be seen the wire from the base that would connect the zinc paint to the circuit.
The X61 was purpose designed as a radio frequency changer.
The input signal would be coupled to the frequency changer via the top cap connection to grid 1. The 6K8 is an equivalent and the data for it shows the top cap connected to g3. In both cases the input signal is connected to the top cap irrespective of the internal construction. This valve has separate triode and hexode sections, whereas the 6K8 employs a clever arrangement of grids.
The mixer at the top. The screening grids are connected together at the top of the supports. Below the centra mica and mounted with the grid axis at right angles to the mixer is the local oscillator triode.
Here the oscillator grid supports can be seen. The central mica is in fact two sheets.
The top mica has fingers to grip the inside of the shouldered envelope. Some of the wire grids can be seen.
The classic envelope is 38 mm in diameter and excluding the IO base pins, is 95 mm tall.
References: Data-sheet, 1043 & 1040. Type X61 was first introduced in 1947. See also1947 adverts.