Psion Plc’s Psion Inc unit says it expects to double US shipments of its palmtop computer to about 6,000 a month from 3,000 by Christmas: the Concord, Massachusetts subsidiary has been selling its Series 3 palmtop computers since 1991 and augmented the line in September with its Series 3a computer, which adds voice capability and other features, and now accounts for the majority of Psion sales worldwide – Psion palmtop shipments have increased to between 21,000 and 22,000 a month from about 15,000 before the latest model arrived.