The VT-238 is the US Government procurement code for the 956 acorn pentode. The acorn valves were designed to have low capacitances and therefore work to higher frequencies than the normal valves. They were designed in the mid 1930's as the quest for efficient working at ever higher frequencies was sought.
The grids can be seen but the image is distorted by the glass. The anode connects to the top pin.
The control grid connection below the main side pins.
The acorn envelope is 14 mm in diameter and, including the base pins, is 45 mm tall.