The 12SN7GT is a small glass tubular audio double triode, the design dates to the 1940's. This was a classic valve with two separate triodes in the one envelope. The 6SN7 six volt version remains the most popular Octal based double triode valve for audio applications. The valve is especially well suited to phase splitter designs.
The makers name and 'Made in USA'.
The control grids are wound on copper and fitted with radiating fins.
The wide glass tube envelope is 29 mm in diameter, and excluding the IO base pins is 62 mm tall.
References: Data-sheet, 1040 & 1043. Type 12SN7GT was first introduced in 1940. See also1940 adverts.