Half-wave rectifier as used at The Marconi Works at Chelmsford in
1920 for experimental broadcasts by Dame Nellie Melba and later
on St. Valentine's Day, February 1922 by 'two emma toc'(2MT) the first
broadcasting station in the U.K., at Marconi's Research
Centre at Writtle.
The example above is early, as before 1930 Marconi were supplying the valves with metal end caps. "2879" on the envelope is thought to be a serial number rather than a batch code.