A 4-pin brass base shell with ceramic insulator, tipped S-shaped envelope. Stamped on envelope: old style RCA logo, Westinghouse logo; on base: "Radiotron / model UV-201 / Patented / Licensed for Amateur / or Experimental Use Only / Manufactured for / Radio Corporation / of America / by / Westinghouse Lamp Co. / U.S.A." References: Tyne, "Saga of the Vacuum Tube," pages 298, 309-315, 360; https://www.radiomuseum.org/tubes/tube_uv201a.html.