IGN 1966 - EPSG:5601

WKT

VERTCS["IGN_1966", 
	VDATUM["IGN_1966"], 
	PARAMETER["Vertical_Shift", 0.0], 
	PARAMETER["Direction", 1.0], 
	UNIT["Meter", 1.0]]

WKT2

VERTCRS["IGN_1966", 
	VDATUM["IGN_1966"], 
	CS[vertical, 1], 
	AXIS["Gravity-related height (H)", up, 
		LENGTHUNIT["Meter", 1.0]], 
	ID["EPSG", "5601"]]

Data from @esri/proj-codes

{
  "wkid": 5601,
  "latestWkid": 5601,
  "macro": "PE_VCS_IGN_1966",
  "name": "IGN_1966",
  "wkt": "VERTCS[\"IGN_1966\",VDATUM[\"IGN_1966\"],PARAMETER[\"Vertical_Shift\",0.0],PARAMETER[\"Direction\",1.0],UNIT[\"Meter\",1.0]]",
  "wkt2": "VERTCRS[\"IGN_1966\",VDATUM[\"IGN_1966\"],CS[vertical,1],AXIS[\"Gravity-related height (H)\",up,LENGTHUNIT[\"Meter\",1.0]],ID[\"EPSG\",\"5601\"]]",
  "description": "IGN 1966",
  "authority": "EPSG",
  "version": "6.15(9.3.0.0)",
  "deprecated": "no",
  "areaname": "French Polynesia - Society Islands - Tahiti",
  "extent": {
    "slat": -17.93,
    "nlat": -17.44,
    "llon": -149.7,
    "rlon": -149.09
  }
}