At the Canadian Museum of Nature, in one of our collection areas, material
acquired in previous years (with the accession number coded for the year
acquired) was catalogued using "1900" for the year portion of the
catalogue number to indicate that the specimens were not acquired in the
year of cataloguing.  In hindsight, this has led to some confusion about
the origin and intention of the change from usual practice.

My advise would be, keep it simple.  If there is a chance someone later
will not understand what you have done, then don't do it.  Have complete
accession files, as the others are suggesting, and all will become clear
to others tracking the data.

Judith