The N18 (direct equivalent = 3Q4 both on the glass) is a miniature dry battery pentode that was designed for portable radio receiver use. The output of 270 mW maximum would only drive a small efficient loudspeaker, not hi-fi.
The paper Osram brand label.
Inside the anode at the top can be seen the tops of the beam plates.
The thin glass tube envelope is 18 mm in diameter and, excluding the B7G base pins, is 45 mm tall.
References: Data-sheet, 3002 & 1040. Type N18 was first introduced in 1950. See also1950 adverts.