The X138 (V888) was the original experimental valve produced in the early 1940s that became the CV138. Post war the data-sheet gives the prototype as the Mazda 6F12 - the commercial version of the earlier valve. See The Murky History article above.
The majority of the original CV138s were made in the US in the mid 1940s.
The valve is a high gain, high impedance sharp cut-off screened pentode. It could be used from AF to VHF up to about 200 MHz originally it was applied to radar.
Looking at one side of the anode. The strap joining the plates has been welded on. Production valves had the nickel sheet stamped to produce the two plates and the joining strap. The 6F12 has this construction. The grids extend wider than the anode.
The electrodes seen along the grid axis. The windings all pass flat across the cathode and each has a different pitch. Above the top mica is a screen.
An oblique view showing the plan sheet anodes that are bright on the inner and outer faces.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 46 mm tall.
Reference: John Wilson & data-sheet. Type X138 was first introduced in 1944.