HVRS71 - EPSG:5610

WKT

VERTCS["HVRS_1971", 
	VDATUM["Croatian_Vertical_Reference_System_1971"], 
	PARAMETER["Vertical_Shift", 0.0], 
	PARAMETER["Direction", 1.0], 
	UNIT["Meter", 1.0]]

WKT2

VERTCRS["HVRS_1971", 
	VDATUM["Croatian_Vertical_Reference_System_1971"], 
	CS[vertical, 1], 
	AXIS["Gravity-related height (H)", up, 
		LENGTHUNIT["Meter", 1.0]], 
	ID["EPSG", "5610"]]

Data from @esri/proj-codes

{
  "wkid": 5610,
  "latestWkid": 5610,
  "macro": "PE_VCS_HVRS_1971",
  "name": "HVRS_1971",
  "wkt": "VERTCS[\"HVRS_1971\",VDATUM[\"Croatian_Vertical_Reference_System_1971\"],PARAMETER[\"Vertical_Shift\",0.0],PARAMETER[\"Direction\",1.0],UNIT[\"Meter\",1.0]]",
  "wkt2": "VERTCRS[\"HVRS_1971\",VDATUM[\"Croatian_Vertical_Reference_System_1971\"],CS[vertical,1],AXIS[\"Gravity-related height (H)\",up,LENGTHUNIT[\"Meter\",1.0]],ID[\"EPSG\",\"5610\"]]",
  "description": "HVRS71",
  "authority": "EPSG",
  "version": "6.15(10.0.0)",
  "deprecated": "no",
  "areaname": "Croatia - onshore",
  "extent": {
    "slat": 42.34,
    "nlat": 46.54,
    "llon": 13.43,
    "rlon": 19.43
  }
}