The 6F5 is classified as a high-μ triode for voltage amplification. It was designed for resistance-capacitance coupled circuits. The 6SF5 is the same valve but the control grid is taken to a base pin not a top cap.
The Sylvania logo and name look to be overprinted on the valve. It is possible that this valve was bought from another maker and re-branded.
The top cap - not easy to make in a metal envelope.
The thin metal tube envelope is 25 mm in diameter and, excluding the IO base pins, is 62 mm tall.
Reference: Data-sheet. Type 6F5 was first introduced in 1940. See also1940 adverts.