BIB-321

[
  {
    "321": {
      "ind1": " ",
      "ind2": " ",
      "subfields": [
        {
          "a": "Annual"
        },
        {
          "b": "1981-"
        }
      ]
    }
  },
  {
    "321": {
      "ind1": " ",
      "ind2": " ",
      "subfields": [
        {
          "a": "Updated irregularly"
        },
        {
          "b": "1984-"
        }
      ]
    }
  }
]

{
  "mainEntity": {
    "replaces": [
      {
        "@type": "Serial",
        "frequency": [
          {
            "@type": "Frequency",
            "label": "Annual",
            "date": "1981-"
          }
        ]
      },
      {
        "@type": "Serial",
        "frequency": [
          {
            "@type": "Frequency",
            "label": "Updated irregularly",
            "date": "1984-"
          }
        ]
      }
    ]
  }
}