De Thuin's work was considered so dangerous to philately that his tools and stock were purchased by the American Philatelic Society in 1966 in order to curtail his activities.
[1] In a 1969 letter to Carl Walske, de Thuin stated that he was a good friend of Angelo Panelli, who was closely associated with the forger Erasmo Oneglia.
[5] According to contemporary Mexican newspaper reports, found by the American Philatelic Society, he first entered Mexico in 1931 as a 'journalist', on the run from French and Belgian police who wanted him for forgery.
[7] De Thuin specialised in forging overprints of Central and South American countries, especially Mexico, thus turning common stamps into rarities.
[9] De Thuin's method of sale was to use one of his aliases to send sheets of approvals to collectors, mainly in the United States, which mixed a few genuine common stamps with forgeries of rarer items.
[12] The numerous aliases were to circumvent fraud orders which prevented the US Post Office from forwarding mail addressed to particular persons or firms.