English and British legal years used to be counted from the accession, not 1 January. “3 Geo. III” is the third year of George III’s reign. The desk covers the Hanoverians to the present, plus a Tudor–Stuart shortlist for silver and portraits.
Accession dates are the conventional ones used in statutes and hallmarks. A year that straddles an accession can legally be “1–2” of the new reign depending on the month — this desk reports the year-number as if the whole calendar year belonged to whoever reigned on 1 July, and notes overlaps. Not a substitute for the Chronological Table of the Statutes.