The 4AV6 is a double diode triode for receiver use. The diodes being for signal detection and AGC rectifier with a maximum current of 2.0 mA. The triode acts as the first audio amplifier and is a high gain type.
The 6AV6 differs only in heater rating and warm-up time. Also made were 3AV6 and 12AV6 types. The 6AT6 was an earlier version.
The triode anode is facing the viewer with the two diode anodes running front to back with the working area above the top mica.
The diode anodes face view.
The thin glass tube envelope is 18 mm in diameter, and excluding the B7G base pins is 48 mm tall.
Reference: Datasheet. Type 4AV6 was first introduced in 1958.