The S11E12 is a special quality version of the 12E1. It is rated as an output valve but it was designed for use as a series or shunt pass device in voltage regulators for instrumentation such as oscilloscopes and radar displays.
The wide glass tube envelope is 38 mm in diameter and, excluding the base pins, is 80 mm tall.
References: Datasheet & 1040. Type S11E12 was first introduced in 1959.