Douglas - EPSG:5750

WKT

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

WKT2

VERTCRS["Douglas", 
	VDATUM["Douglas"], 
	CS[vertical, 1], 
	AXIS["Gravity-related height (H)", up, 
		LENGTHUNIT["Meter", 1.0]], 
	ID["EPSG", "5750"]]

Data from @esri/proj-codes

{
  "wkid": 5750,
  "latestWkid": 5750,
  "macro": "PE_VCS_DOUGLAS",
  "name": "Douglas",
  "wkt": "VERTCS[\"Douglas\",VDATUM[\"Douglas\"],PARAMETER[\"Vertical_Shift\",0.0],PARAMETER[\"Direction\",1.0],UNIT[\"Meter\",1.0]]",
  "wkt2": "VERTCRS[\"Douglas\",VDATUM[\"Douglas\"],CS[vertical,1],AXIS[\"Gravity-related height (H)\",up,LENGTHUNIT[\"Meter\",1.0]],ID[\"EPSG\",\"5750\"]]",
  "description": "Douglas",
  "authority": "EPSG",
  "version": "6.15(9.3.0.0)",
  "deprecated": "no",
  "areaname": "Isle of Man",
  "extent": {
    "slat": 54.02,
    "nlat": 54.44,
    "llon": -4.87,
    "rlon": -4.27
  }
}