Ha Tien 1960 - EPSG:5726

WKT

VERTCS["Ha_Tien_1960", 
	VDATUM["Ha_Tien_1960"], 
	PARAMETER["Vertical_Shift", 0.0], 
	PARAMETER["Direction", 1.0], 
	UNIT["Meter", 1.0]]

WKT2

VERTCRS["Ha_Tien_1960", 
	VDATUM["Ha_Tien_1960"], 
	CS[vertical, 1], 
	AXIS["Gravity-related height (H)", up, 
		LENGTHUNIT["Meter", 1.0]], 
	ID["EPSG", "5726"]]

Data from @esri/proj-codes

{
  "wkid": 5726,
  "latestWkid": 5726,
  "macro": "PE_VCS_HA_TIEN_1960",
  "name": "Ha_Tien_1960",
  "wkt": "VERTCS[\"Ha_Tien_1960\",VDATUM[\"Ha_Tien_1960\"],PARAMETER[\"Vertical_Shift\",0.0],PARAMETER[\"Direction\",1.0],UNIT[\"Meter\",1.0]]",
  "wkt2": "VERTCRS[\"Ha_Tien_1960\",VDATUM[\"Ha_Tien_1960\"],CS[vertical,1],AXIS[\"Gravity-related height (H)\",up,LENGTHUNIT[\"Meter\",1.0]],ID[\"EPSG\",\"5726\"]]",
  "description": "Ha Tien 1960",
  "authority": "EPSG",
  "version": "6.15(9.3.0.0)",
  "deprecated": "no",
  "areaname": "Asia - Cambodia and Vietnam - mainland",
  "extent": {
    "slat": 8.33,
    "nlat": 23.4,
    "llon": 102.14,
    "rlon": 109.53
  }
}