The 6BL7GT is a purpose designed television frame generator and frame output valve for wide deflection tubes and 525 line systems. Each section can dissipate 10 Watts.
The two anodes have the same orientation and thus we can see through the stitches of both sections. The grids are supplied with heat radiating fins.
The dome with identification.
The wide glass tube envelope is 28 mm in diameter, and excluding the IO base pins is 67 mm tall.
Reference: Data-sheet. Type 6BL7GT was first introduced in 1950. See also1950 adverts.