NMVD03 height - EPSG:6640

WKT

VERTCS["NMVD03_height", 
	VDATUM["Northern_Marianas_Vertical_Datum_of_2003"], 
	PARAMETER["Vertical_Shift", 0.0], 
	PARAMETER["Direction", 1.0], 
	UNIT["Meter", 1.0]]

WKT2

VERTCRS["NMVD03_height", 
	VDATUM["Northern_Marianas_Vertical_Datum_of_2003"], 
	CS[vertical, 1], 
	AXIS["Gravity-related height (H)", up, 
		LENGTHUNIT["Meter", 1.0]], 
	ID["EPSG", "6640"]]

Data from @esri/proj-codes

{
  "wkid": 6640,
  "latestWkid": 6640,
  "macro": "PE_VCS_NMVD03_HT",
  "name": "NMVD03_height",
  "wkt": "VERTCS[\"NMVD03_height\",VDATUM[\"Northern_Marianas_Vertical_Datum_of_2003\"],PARAMETER[\"Vertical_Shift\",0.0],PARAMETER[\"Direction\",1.0],UNIT[\"Meter\",1.0]]",
  "wkt2": "VERTCRS[\"NMVD03_height\",VDATUM[\"Northern_Marianas_Vertical_Datum_of_2003\"],CS[vertical,1],AXIS[\"Gravity-related height (H)\",up,LENGTHUNIT[\"Meter\",1.0]],ID[\"EPSG\",\"6640\"]]",
  "description": "NMVD03 height",
  "authority": "EPSG",
  "version": "8.2.10(10.3.1)",
  "deprecated": "no",
  "areaname": "Northern Mariana Islands - Rota, Saipan and Tinian",
  "extent": {
    "slat": 14.06,
    "nlat": 15.35,
    "llon": 145.06,
    "rlon": 145.89
  }
}