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The idea of marking the money card with a paper clip, then secretly switching the clip to another card, is Bill Woodfield's and was published in Genii, Vol. 15 No. 6, Feb. 1951, p. 234. It is also mentioned in The Phoenix, No. 258, June 27 1952, p. 1030, and by Audley Walsh in Gen, Vol. 8 No. 3, July 1952, p. 76.