Windows exist in rear gables of bob lofts at Talargoch Lead Mine, Dyserth, Clwyd, 100" pumping engine house of 1860, Penprys Colliery, Llanelli, Carmarthenshire, whim engine house of 1889. Based on the photos of rear gables of Cornish engine houses in Wales that I have to hand, this is a rate of around one in three.