There are a number of varieties of the stamps you refer to. The "1996" dated ones would be Scott #2907 (reprinted from the earlier versions.)
There's also Scott #2602 that is easily identifiable, because the "USA" is at the end of the text line rather than at the beginning and the colors of the text are switched ("USA" is red and "Bulk Rate" is blue).
As for Scott #2603 and #2604, here's a side-by-side comparison:
