The 6SC7 is a metal enveloped double triode, these double triodes have been used in guitar amplifiers and the ECC83 is used for upgrades although the 6SL7 was the first successor valve in the early 1940's.
This example from General Electric looks to be new and unused with perfect matt balck paint.
We also have the 12SC7 that differs only in heater rating.
Made in USA.
The General Electric logo and six dots.
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 6SC7 was first introduced in 1938.