The Nuvistor from RCA was a triode for use at VHF and build into a metal case just larger than a transistor of the time. The base is ceramic sealed to the metal case and five fine pins, on a 12 pin grid, form the connections and the electrode supports internally.
The picture above shows the special twelvar base.
The Nuvistor is built as a series of co-axial electrodes but they are all very small, and small inter electrode capacitances result. Thus the high frequency of operation. Unlike the transistors of the time the Nuvistor had the overload and intermodulation characteristics of the valve, superior to the transistor.
The thin metal tube envelope is 9 mm in diameter (10 mm maximum), and excluding the base pins, is 20 mm tall. The pins are protected by the extended case, that also acts as a guide into the base for the fragile pins.
Reference: Data-sheet. Type 7895 was first introduced in 1961.