The 12SC7 is a metal enveloped double triode, these valves have been used in guitar amplifiers and the ECC83 is used for upgrades.
We also have the 6SC7 that differs only in heater rating.
The valve is labeled Marconi and has the RVC logo. The Radio Valve Company of Canada was created in 1922 as a joint venture between Canadian General Electric and Marconi.
The RVC logo.
The thin metal tube envelope is 25 mm in diameter, and excluding the IO base pins is 51 mm tall.
References: Data-sheet, 1043, 3002 & 1040. Type 12SC7 was first introduced in 1939. See also1939 adverts.