Great Britain, "London, Mar 17, 2-30P, 1898, England" Involute Flag Cancel. Complete strike tying 1887, ½d vermilion on cover to Rotherhithe, receiving backstamp; allover cover toning, still a Very Fine strike, a Great Pioneer Machine Cancel Rarity, With This Being the Unique on Cover Example of The London Experimental Flag Machine Postmark. SG No. 197  Estimate $1,000 - 1,500. Mr. Martin V.B. Ethridge approached the British Post Office in 1898 and attempted to sell them his rapid postmarking machine manufactured by his new company, Empire Canceling Machine Company. Trials for this machine were held only four days, March 15, 17, 18 and 21, 1898, for one hour each day with only 3,200 pieces of mail canceled. This is Britain's only pioneer machine cancel in her flag's design ever to be used on mail. Four other examples are known of this postmark are known - one on a postal card, one on a cover front and two cutouts from envelopes. As this is the only complete and intact, receipt marked survivor of the only usage of a flag cancel in any land's history, writer's have called this "The Rarest Flag Cancel in the World". . $0 (Image)