The Flying Beetle

Originally released serially in two issues of Boys Own Paper, October and November, 1926. Released in large-sized paperback format by Amalgamated Press in 1928. Released in book format in 1935.
"A figure ... surprised the six masked conspirators plotting the downfall of England. One by one he unmasked them and informed them that one by one they would die. Who was this mysterious intruder? All that the conspirators knew was that he left a card embossed with the replica of a black flying beetle."