We notice that MathSciNet describes this paper as Research Paper 57 with a comment "Paper number later changed by the publisher from 57 to 60".
From the journal's point of view, the paper number was always R60. The pdf that can be downloaded from our website has always described the paper as R60. It has not been changed since it was published on 13 September 2004. The date stamp on the file confirms this. We would ask authors to cite the paper as R60.