The 7S7 triode heptode was designed for frequency changer use in radio receivers. The 7S7 was also made for mobile use with 12 Volt vehicle batteries for the heater, in this case the valve was called the 14S7.
The Brimar identification is clear but the coating on the inside of the envelope partly obscures the details of the valve's construction. Close inspection under strong light reveals a single cathode and two valve sections mounted vertically.
The dome allows some view of the construction. The top section is the triode.
The wide glass tube envelope is 28 mm in diameter and, excluding the B8B base pins, is 57 mm tall.
References: Data-sheet & 1040. Type 7S7 was first introduced in 1948.