The following excerpt is from the New World Dictionary , 2nd College edition (Simon & Schuster, 1984), p.xxxiv:

"It is especially in the OED Supplement (1933) that the labeling of United States terms and meanings becomes particularly noticeable. The reason for this was that the major part of Sir William Craigie's work on the Supplement was done while he was in this country and could draw on the increasingly large amount of American evidence he and his students were collecting in Chicago. The practice so well carried out in the OED of identifying United States terms and meanings was likewise followed by the Century Dictionary and continued as well in the large single volume dictionaries. The contributions made by these larger general dictionaries, added to what has been made available in smaller works of a more restricted scope, have increased greatly the evidence now available for distinguishing between English and what has been called from early times American English."