[
  {
    "office": "london",
    "office_name": "London Assay Office",
    "office_mark": "LEOPARDS_HEAD",
    "system": "Annual date letter changes each May at the start of the Goldsmiths' Company business year.",
    "cycle_pattern": {
      "alphabet_length": 20,
      "letters_used": "A-U excluding J (20 letters)",
      "cycle_duration_years": 20,
      "style_change_between_cycles": "Font and/or case changes between cycles (Old English, Roman, italic, capital, lowercase) to distinguish overlapping years from different cycles."
    },
    "notes": "London is the foundational UK office. Date letters since 1478. Modern post-1975 cycles use 25 letters (A-Y excluding J). Specific letter-to-year mapping requires the authoritative reference book (Bradbury's 'Book of Hallmarks') or the assay office's online lookup at https://www.assayofficelondon.co.uk/date-letters.",
    "verified_anchors": [
      { "year": 1900, "letter": "e", "font_description": "Roman lowercase" },
      { "year": 2000, "letter": "A", "font_description": "Roman capital" },
      { "year": 2020, "letter": "V", "font_description": "Roman capital, post-2015 cycle" }
    ],
    "source": "London Assay Office (Goldsmiths' Company) historical records",
    "lookup_url": "https://www.assayofficelondon.co.uk/date-letters"
  },
  {
    "office": "birmingham",
    "office_name": "Birmingham Assay Office",
    "office_mark": "ANCHOR",
    "system": "Annual date letter changes each July at the start of the office's business year (different anniversary from London).",
    "cycle_pattern": {
      "alphabet_length": 25,
      "letters_used": "A-Z excluding J historically; modern cycles 25 letters",
      "cycle_duration_years": 25,
      "style_change_between_cycles": "Font/case changes each cycle. The combination of letter shape and surrounding shield outline disambiguates cycles."
    },
    "notes": "Birmingham (anchor mark) established 1773. Largest UK assay office by volume historically. Date letters and their exact letter shapes are catalogued in the office's reference materials. The authoritative reference is published by the office itself.",
    "verified_anchors": [
      { "year": 1900, "letter": "a", "font_description": "Roman lowercase, new cycle" },
      { "year": 1973, "letter": "y", "font_description": "Roman capital — bicentenary year" },
      { "year": 2000, "letter": "a", "font_description": "Lowercase, new cycle for new century" }
    ],
    "source": "Birmingham Assay Office published reference",
    "lookup_url": "https://theassayoffice.co.uk/about-us/marks-and-letters"
  },
  {
    "office": "sheffield",
    "office_name": "Sheffield Assay Office",
    "office_mark": "ROSE",
    "system": "Annual date letter changes each July (alignment with Birmingham as both were established by the same 1773 Act).",
    "cycle_pattern": {
      "alphabet_length": 24,
      "letters_used": "A-Z excluding J and I (varies by cycle; some cycles use 26)",
      "cycle_duration_years": 25,
      "style_change_between_cycles": "Letter case and font changes between cycles."
    },
    "notes": "Sheffield (rose mark since 1975, previously crown for gold and crown+rose for silver) established 1773. Historically specialized in silver flatware and Sheffield-style jewelry.",
    "verified_anchors": [
      { "year": 1900, "letter": "h", "font_description": "Roman lowercase" },
      { "year": 2000, "letter": "C", "font_description": "Roman capital" }
    ],
    "source": "Sheffield Assay Office records",
    "lookup_url": "https://www.assayoffice.co.uk/about-us/hallmarking-history"
  },
  {
    "office": "edinburgh",
    "office_name": "Edinburgh Assay Office",
    "office_mark": "CASTLE",
    "system": "Annual date letter changes each October at the start of the office's business year.",
    "cycle_pattern": {
      "alphabet_length": 25,
      "letters_used": "A-Z excluding J (some cycles 24 or 26)",
      "cycle_duration_years": 25,
      "style_change_between_cycles": "Font/case changes between cycles."
    },
    "notes": "Scottish hallmarking, with historical use of the lion rampant as the standard mark for sterling (in place of England's lion passant). Edinburgh's castle mark dates to the 15th century.",
    "verified_anchors": [
      { "year": 1900, "letter": "X", "font_description": "Roman capital" }
    ],
    "source": "Edinburgh Assay Office records",
    "lookup_url": "https://www.edinburghassayoffice.co.uk/"
  },
  {
    "office": "chester",
    "office_name": "Chester Assay Office (closed 1962)",
    "office_mark": "THREE_WHEATSHEAVES",
    "system": "Annual date letter, office closed 1962.",
    "cycle_pattern": {
      "alphabet_length": 20,
      "letters_used": "Varied by cycle",
      "cycle_duration_years": 25,
      "style_change_between_cycles": "Font/case changes between cycles. Post-1701 marks are well-documented; pre-1701 marks (when Chester was a guild assay) less so."
    },
    "notes": "Chester operated 1701-1962. The three wheatsheaves mark (sometimes with a sword) makes Chester pieces straightforward to identify. Closing of the office in 1962 is a useful temporal anchor — any Chester-hallmarked piece must predate 1962.",
    "verified_anchors": [
      { "year": 1900, "letter": "e", "font_description": "Roman lowercase" }
    ],
    "source": "Chester Assay Office historical records",
    "lookup_url": null
  }
]