The 6GY6 is a dual control pentode for use in television receivers. The design function is as a gated AGC amplifier.
The RCA identification.
The RCA logo and within the notch can be seen the small pitch of the suppressor grid. This tightly wound grid is then effective in controlling the electron flow - hence dual control.
The thin glass tube envelope is 18 mm in diameter and, excluding the base pins, is 47 mm tall.
Reference: Data-sheet. Type 6GY6 was first introduced in 1961. See also1961 adverts.