EPSG:4033 - Unknown datum based upon the OSU91A ellipsoid

{
  "$schema": "https://proj.org/schemas/v0.7/projjson.schema.json",
  "type": "GeographicCRS",
  "name": "Unknown datum based upon the OSU91A ellipsoid",
  "datum": {
    "type": "GeodeticReferenceFrame",
    "name": "Not specified (based on OSU91A ellipsoid)",
    "ellipsoid": {
      "name": "OSU91A",
      "semi_major_axis": 6378136.3,
      "inverse_flattening": 298.257223563
    }
  },
  "coordinate_system": {
    "subtype": "ellipsoidal",
    "axis": [
      {
        "name": "Geodetic latitude",
        "abbreviation": "Lat",
        "direction": "north",
        "unit": "degree"
      },
      {
        "name": "Geodetic longitude",
        "abbreviation": "Lon",
        "direction": "east",
        "unit": "degree"
      }
    ]
  },
  "scope": "Not recommended.",
  "area": "Not specified.",
  "bbox": {
    "south_latitude": -90,
    "west_longitude": -180,
    "north_latitude": 90,
    "east_longitude": 180
  },
  "id": {
    "authority": "EPSG",
    "code": 4033
  }
}

{'crs_wkt': 'GEOGCRS["Unknown datum based upon the OSU91A '
            'ellipsoid",DATUM["Not specified (based on OSU91A '
            'ellipsoid)",ELLIPSOID["OSU91A",6378136.3,298.257223563,LENGTHUNIT["metre",1]]],PRIMEM["Greenwich",0,ANGLEUNIT["degree",0.0174532925199433]],CS[ellipsoidal,2],AXIS["geodetic '
            'latitude '
            '(Lat)",north,ORDER[1],ANGLEUNIT["degree",0.0174532925199433]],AXIS["geodetic '
            'longitude '
            '(Lon)",east,ORDER[2],ANGLEUNIT["degree",0.0174532925199433]],USAGE[SCOPE["Not '
            'recommended."],AREA["Not '
            'specified."],BBOX[-90,-180,90,180]],ID["EPSG",4033]]',
 'geographic_crs_name': 'Unknown datum based upon the OSU91A ellipsoid',
 'grid_mapping_name': 'latitude_longitude',
 'horizontal_datum_name': 'Not specified (based on OSU91A ellipsoid)',
 'inverse_flattening': 298.257223563,
 'longitude_of_prime_meridian': 0.0,
 'prime_meridian_name': 'Greenwich',
 'reference_ellipsoid_name': 'OSU91A',
 'semi_major_axis': 6378136.3,
 'semi_minor_axis': 6356751.616592146}