RH1900 - EPSG:5615

WKT

VERTCS["RH1900", 
	VDATUM["Rikets_Hojdsystem_1900"], 
	PARAMETER["Vertical_Shift", 0.0], 
	PARAMETER["Direction", 1.0], 
	UNIT["Meter", 1.0]]

WKT2

VERTCRS["RH1900", 
	VDATUM["Rikets_Hojdsystem_1900"], 
	CS[vertical, 1], 
	AXIS["Gravity-related height (H)", up, 
		LENGTHUNIT["Meter", 1.0]], 
	ID["EPSG", "5615"]]

Data from @esri/proj-codes

{
  "wkid": 5615,
  "latestWkid": 5615,
  "macro": "PE_VCS_RH1900",
  "name": "RH1900",
  "wkt": "VERTCS[\"RH1900\",VDATUM[\"Rikets_Hojdsystem_1900\"],PARAMETER[\"Vertical_Shift\",0.0],PARAMETER[\"Direction\",1.0],UNIT[\"Meter\",1.0]]",
  "wkt2": "VERTCRS[\"RH1900\",VDATUM[\"Rikets_Hojdsystem_1900\"],CS[vertical,1],AXIS[\"Gravity-related height (H)\",up,LENGTHUNIT[\"Meter\",1.0]],ID[\"EPSG\",\"5615\"]]",
  "description": "RH1900",
  "authority": "EPSG",
  "version": "6.15(10.0.0)",
  "deprecated": "no",
  "areaname": "Sweden - onshore",
  "extent": {
    "slat": 55.28,
    "nlat": 69.07,
    "llon": 10.93,
    "rlon": 24.17
  }
}