Just found these pages at the Valve Museum, which I didn't know existed. It turns out that American equivalent valves including 6F6G (=KT63), 6J7G (=Z63/KTZ63), 6L7G (=X64) were around in 1936
http://www.r-type.org/inx/inx1936.htm but the Marconi numbers used in this set do not seem to have appeared until 1937
http://www.r-type.org/inx/inx1937.htm and so Paul is probably right
Cheers, Jerry