MGI 3 Degree Gauss Zone 6 - EPSG:31266

WKT

PROJCS["MGI_3_Degree_Gauss_Zone_6", 
	GEOGCS["GCS_MGI", 
		DATUM["D_MGI", 
			SPHEROID["Bessel_1841", 6377397.155, 299.1528128]], 
		PRIMEM["Greenwich", 0.0], 
		UNIT["Degree", 0.0174532925199433]], 
	PROJECTION["Gauss_Kruger"], 
	PARAMETER["False_Easting", 6500000.0], 
	PARAMETER["False_Northing", 0.0], 
	PARAMETER["Central_Meridian", 18.0], 
	PARAMETER["Scale_Factor", 1.0], 
	PARAMETER["Latitude_Of_Origin", 0.0], 
	UNIT["Meter", 1.0]]

WKT2

PROJCRS["MGI_3_Degree_Gauss_Zone_6", 
	BASEGEOGCRS["GCS_MGI", 
		DATUM["D_MGI", 
			ELLIPSOID["Bessel_1841", 6377397.155, 299.1528128]], 
		PRIMEM["Greenwich", 0.0], 
		CS[ellipsoidal, 2], 
		AXIS["Latitude (lat)", north, 
			ORDER[1]], 
		AXIS["Longitude (lon)", east, 
			ORDER[2]], 
		ANGLEUNIT["Degree", 0.0174532925199433]], 
	CONVERSION["Gauss_Kruger", 
		METHOD["Gauss_Kruger"], 
		PARAMETER["False_Easting", 6500000.0], 
		PARAMETER["False_Northing", 0.0], 
		PARAMETER["Central_Meridian", 18.0], 
		PARAMETER["Scale_Factor", 1.0], 
		PARAMETER["Latitude_Of_Origin", 0.0]], 
	CS[Cartesian, 2], 
	AXIS["Easting (X)", east, 
		ORDER[1]], 
	AXIS["Northing (Y)", north, 
		ORDER[2]], 
	LENGTHUNIT["Meter", 1.0], 
	ID["EPSG", "31266"]]

Data from @esri/proj-codes

{
  "wkid": 31266,
  "latestWkid": 31266,
  "gcsWkid": 4312,
  "macro": "PE_PCS_MGI_3_DEGREE_GAUSS_6",
  "name": "MGI_3_Degree_Gauss_Zone_6",
  "wkt": "PROJCS[\"MGI_3_Degree_Gauss_Zone_6\",GEOGCS[\"GCS_MGI\",DATUM[\"D_MGI\",SPHEROID[\"Bessel_1841\",6377397.155,299.1528128]],PRIMEM[\"Greenwich\",0.0],UNIT[\"Degree\",0.0174532925199433]],PROJECTION[\"Gauss_Kruger\"],PARAMETER[\"False_Easting\",6500000.0],PARAMETER[\"False_Northing\",0.0],PARAMETER[\"Central_Meridian\",18.0],PARAMETER[\"Scale_Factor\",1.0],PARAMETER[\"Latitude_Of_Origin\",0.0],UNIT[\"Meter\",1.0]]",
  "wkt2": "PROJCRS[\"MGI_3_Degree_Gauss_Zone_6\",BASEGEOGCRS[\"GCS_MGI\",DATUM[\"D_MGI\",ELLIPSOID[\"Bessel_1841\",6377397.155,299.1528128]],PRIMEM[\"Greenwich\",0.0],CS[ellipsoidal,2],AXIS[\"Latitude (lat)\",north,ORDER[1]],AXIS[\"Longitude (lon)\",east,ORDER[2]],ANGLEUNIT[\"Degree\",0.0174532925199433]],CONVERSION[\"Gauss_Kruger\",METHOD[\"Gauss_Kruger\"],PARAMETER[\"False_Easting\",6500000.0],PARAMETER[\"False_Northing\",0.0],PARAMETER[\"Central_Meridian\",18.0],PARAMETER[\"Scale_Factor\",1.0],PARAMETER[\"Latitude_Of_Origin\",0.0]],CS[Cartesian,2],AXIS[\"Easting (X)\",east,ORDER[1]],AXIS[\"Northing (Y)\",north,ORDER[2]],LENGTHUNIT[\"Meter\",1.0],ID[\"EPSG\",\"31266\"]]",
  "description": "MGI 3 Degree Gauss Zone 6",
  "authority": "EPSG",
  "version": "4.5(8.1.2)",
  "deprecated": "no",
  "areaname": "Europe - former Yugoslavia onshore 16.5~E to 19.5~E",
  "extent": {
    "slat": 41.79,
    "nlat": 46.55,
    "llon": 16.5,
    "rlon": 19.51
  }
}