"letter stock" meaning in English

{
  "forms": [
    {
      "form": "letter stocks",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
        "1": "~"
      },
      "expansion": "letter stock (countable and uncountable, plural letter stocks)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        {
          "kind": "other",
          "name": "English entries with incorrect language header",
          "parents": [
            "Entries with incorrect language header",
            "Entry maintenance"
          ],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Pages with 1 entry",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Pages with entries",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "topical",
          "langcode": "en",
          "name": "Finance",
          "orig": "en:Finance",
          "parents": [
            "Business",
            "Economics",
            "Society",
            "Social sciences",
            "All topics",
            "Sciences",
            "Fundamental"
          ],
          "source": "w"
        }
      ],
      "glosses": [
        "Synonym of tracking stock"
      ],
      "id": "en-letter_stock-en-noun-qqPJewgQ",
      "links": [
        [
          "finance",
          "finance#Noun"
        ],
        [
          "tracking stock",
          "tracking stock#English"
        ]
      ],
      "raw_glosses": [
        "(finance) Synonym of tracking stock"
      ],
      "synonyms": [
        {
          "tags": [
            "synonym",
            "synonym-of"
          ],
          "word": "tracking stock"
        }
      ],
      "tags": [
        "countable",
        "uncountable"
      ],
      "topics": [
        "business",
        "finance"
      ]
    }
  ],
  "word": "letter stock"
}

{
  "forms": [
    {
      "form": "letter stocks",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {
        "1": "~"
      },
      "expansion": "letter stock (countable and uncountable, plural letter stocks)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "categories": [
        "English countable nouns",
        "English entries with incorrect language header",
        "English lemmas",
        "English multiword terms",
        "English nouns",
        "English uncountable nouns",
        "Pages with 1 entry",
        "Pages with entries",
        "en:Finance"
      ],
      "glosses": [
        "Synonym of tracking stock"
      ],
      "links": [
        [
          "finance",
          "finance#Noun"
        ],
        [
          "tracking stock",
          "tracking stock#English"
        ]
      ],
      "raw_glosses": [
        "(finance) Synonym of tracking stock"
      ],
      "synonyms": [
        {
          "tags": [
            "synonym",
            "synonym-of"
          ],
          "word": "tracking stock"
        }
      ],
      "tags": [
        "countable",
        "uncountable"
      ],
      "topics": [
        "business",
        "finance"
      ]
    }
  ],
  "word": "letter stock"
}