I believe the reason our artifacts used “retro numbers” instead of the year they were renumbered is so the year the donation was made would be reflected in the number. I cannot be sure since this was done many years ago, but they were trying to fit the objects into the new scheme and that meant associating the catalog number with the donation year.
Molly-I don’t think it is bad practice. It is not traditional, but all museums use their numbering systems in different ways. If Ashely decides it is too much work and too much confusion to assign a completely new number, adding letters to the pieces is not unheard of. I have seen this at many of the institutions I have worked at. It just depends on how your organization wishes to create their cataloging system.