Gas-filled. Initially filled with nitrogen to 0.06 mm mercury. From September 1917 on helium was used at a pressure of 0.6 mm mercury. The lettering on the envelope reads: Valve wireless receiving R.2 Pat. No. ?437. A6335. The other side reads: Osram GEC Made in England. R.2. Produced by BTH, Osram (GEC) and Ediswan. See also: R4