The VT-131 is the commercial 12SK7 supplied to the American government. It is a variable μ pentode for RF and IF amplifiers in superhet receivers. The gain of the device being controlled by a negative voltage derived from rectifying the final IF signal. In this way strong signals do not overload the receive chain because the various controlled stages have their gain reduced.
The 12.6 Volt heater was introduced so that the heater could be run directly from a vehicle battery. It would have been common to find these valves in professional/military mobile equipment. For fixed station use the 6SK7 was an identical device in all other respects.
The 'S' denotes a single ended device without top cap. The 6K7 is the original version of the design and the 12K7 would be the 12 Volt heater version.
The Radiotron brand name and Made in USA.
The RCA logo
The thin metal tube envelope is 25 mm in diameter and, excluding the IO base pins, is 51 mm tall.
References: Data-sheet & 1040. Type VT-131 was first introduced in 1939.