The VT-121 is an acorn triode valve and is the American service type designation for the RCA type 955 also seen on the glass.
The acorn valves were the product of RCA research into making high frequency valves. The requirement was to make a linear reduction in all dimensions so as to reduce the inter-electrode capacitancies. The acorn types had about half of the capacitancies of other valves of the mid 1930s and could operate at up to 400 MHz. The 955/VT-121 was the first acorn miniature valve to go into production, this was in 1934. The acorn valves had indirectly heated cathodes the heaters were rated at 6.3 Volts and consumed 0.15 Amps. The heater and cathode assembly was about half the length and the same diameter as a household pin.
This picture shows the anode assembly.
The connecting wires to the heater can be clearly seen.
The acorn envelope is 14 mm in diameter and, excluding the pins is 33 mm tall.
References: 1004. Type VT-121 was first introduced in 1934.