The American Government code of VT-209 is also the commercial 12SG7. It is a 12.6 Volt version of the 6SG7 semi-vari mu pentode except for the heater they are identical.
The other side has the commercial designation of 12SG7.
The RCA logo.
The thin metal tube envelope is 25 mm diameter, and excluding the IO base pins, is 51 mm tall.
References: Data-sheet, 1040 & 1043. Type VT-209 was first introduced in 1940. See also1940 adverts.