There's a note on the table of releases for 2004 that reads:
As an open edition, the first year of production pieces should have the prefix letter "A" before the edition (serial) number. However, the first pieces released of Hooper Strait were numbered 1201 to 1500 without the letter. Others in the production run have the prefix letter "A".
There was also a similar numbering error with the Hereford Inlet Fall - some of which had "A" when they shouldn't have.
