Dear Beth H.,
I was darn close to this solution but it did not dawn on me that the sorting in Mac OSX might not work with the format 'CommonName_##' but does it only correctly with the #s in front, i.e. '##_CommonName'. Incidentally, although I did the ordering of the filenames with my assumed solution in finder, PSE did not strictly follow that ordering.
Of course, everything has its own logic, like the religions.
Thank you very much for you very quick response.