The 7851 is a miniature electrometer screen grid tetrode. Unlike the 6BS7, designed for valve voltmeters or audio, the 7851 is specifically designed for electrometer applications alone.
From the data-sheet: Its high μ triode distinguishes it from conventional electrometer valves and makes it particularly useful as an very high gain current amplifier in applications where the input signal is in the order of pico Amps. Careful shielding from light as well as magnetic fields is required as would be a very carefully controlled heater voltage.
The screen extends to enclose the whole space between the top and bottom micas, thus obscuring the internal electrodes.
The Tung-Sol name and ade in USA is printed onto the glass.
Thanks to R J Sutherland for the excellent images and information.
The thin glass tube envelope is 18 mm in diameter, and excluding the B7G base pins is 40 mm tall.
Reference: Datasheet. Type 7851 was first introduced in 1961.