Madrid 1870 (Madrid) - EPSG:4903

WKT

GEOGCS["GCS_Madrid_1870_Madrid", 
	DATUM["D_Madrid_1870", 
		SPHEROID["Struve_1860", 6378298.3, 294.73]], 
	PRIMEM["Madrid", -3.687375], 
	UNIT["Degree", 0.0174532925199433]]

WKT2

GEOGCRS["GCS_Madrid_1870_Madrid", 
	DATUM["D_Madrid_1870", 
		ELLIPSOID["Struve_1860", 6378298.3, 294.73]], 
	PRIMEM["Madrid", -3.687375, 
		ANGLEUNIT["Degree", 0.0174532925199433]], 
	CS[ellipsoidal, 2], 
	AXIS["Latitude (lat)", north, 
		ORDER[1]], 
	AXIS["Longitude (lon)", east, 
		ORDER[2]], 
	ANGLEUNIT["Degree", 0.0174532925199433], 
	ID["EPSG", "4903"]]

Data from @esri/proj-codes

{
  "wkid": 4903,
  "latestWkid": 4903,
  "macro": "PE_GCS_MADRID_1870_MADRID",
  "name": "GCS_Madrid_1870_Madrid",
  "wkt": "GEOGCS[\"GCS_Madrid_1870_Madrid\",DATUM[\"D_Madrid_1870\",SPHEROID[\"Struve_1860\",6378298.3,294.73]],PRIMEM[\"Madrid\",-3.687375],UNIT[\"Degree\",0.0174532925199433]]",
  "wkt2": "GEOGCRS[\"GCS_Madrid_1870_Madrid\",DATUM[\"D_Madrid_1870\",ELLIPSOID[\"Struve_1860\",6378298.3,294.73]],PRIMEM[\"Madrid\",-3.687375,ANGLEUNIT[\"Degree\",0.0174532925199433]],CS[ellipsoidal,2],AXIS[\"Latitude (lat)\",north,ORDER[1]],AXIS[\"Longitude (lon)\",east,ORDER[2]],ANGLEUNIT[\"Degree\",0.0174532925199433],ID[\"EPSG\",\"4903\"]]",
  "description": "Madrid 1870 (Madrid)",
  "authority": "EPSG",
  "version": "4.5(8.0.1)",
  "deprecated": "no",
  "areaname": "Spain - mainland onshore",
  "extent": {
    "slat": 35.95,
    "nlat": 43.82,
    "llon": -9.37,
    "rlon": 3.39
  }
}