A development by Sylvania from 1937 but published in 1940.
To reduce the inherent input and output capacitance of a valve, particularly for handling ultra-short waves, the base of the bulb is sealed off by a flat 'header' of glass through which metal contact prongs pass. The inner ends of the prongs are connected to the electrodes inside the bulb, whilst the outer ends are plugged directly into the usual socket or valveholder.
To supplement the effect of the screening grid and to ensure a more complete electrostatic isolation of the electrodes, a metal plate is fitted around the exterior of the flat glass base, and is perforated to allow the contact prongs to pass through.
Hygrade Sylvania Corp. Convention date (USA) May 28th, 1937. N0. 516110.