The Read Codes Four Byte Set:

{
  "description" : "The Read Codes Four Byte Set consists of 4 alphanumeric characters. This version contains approximately 40,000 codes arranged in a hierarchical structure.\r\n\r\nTop level hierarchy sections: Disorders Findings Surgical procedures Investigations Occupations Drugs",
  "date" : "2021-11-09T10:00:00+10:00",
  "publisher" : "TBD - External Body",
  "content" : "not-present",
  "property" : null,
  "name" : "RcFB",
  "experimental" : false,
  "resourceType" : "CodeSystem",
  "title" : "The Read Codes Four Byte Set:",
  "extension" : [ {
    "url" : "http://health-samurai.io/extensions/source",
    "valueCode" : "db"
  } ],
  "status" : "retired",
  "id" : "a84d6c1f-8c3a-4ae6-b960-e8308e57a517",
  "url" : "http://terminology.hl7.org/CodeSystem/rcFB",
  "identifier" : [ {
    "system" : "urn:ietf:rfc:3986",
    "value" : "urn:oid:2.16.840.1.113883.6.28"
  } ],
  "version" : "2.0.0",
  "contact" : [ {
    "name" : "Paul Frosdick, NHS Information Authority"
  } ],
  "text" : {
    "div" : "<div xmlns=\"http://www.w3.org/1999/xhtml\"><p>This code system http://terminology.hl7.org/CodeSystem/rcFB defines many codes, but they are not represented here</p></div>",
    "status" : "generated"
  }
}