The 14S7 is a triode heptode frequency changer from 1948. The American valves with designations starting with 14 were designed for operation on nominal 12 Volt vehicle supplies. The 7S7 has identical operating characteristics but with a standard 6.3 Volt heater.
The B8B or Loctal base as equi-spaced pins and so a locating spigot with key and a locating pip on the metal skirt are essential. The dark cylinder is the electrostatic screen, this screen extends down to the base to shield the anode pin.
Some of the helical wire grids.
The wide glass tube envelope is 29 mm in diameter and, excluding the B8B base pins, is 58 mm tall.
References: Data-sheet & 1040. Type 14S7 was first introduced in 1948.