Historical Directories has a couple of references to a Spittal Mill for Pemb's -
Kelly's Directory Monmouthshire + S Wales 1895 Pt2 "Davies, Lewis, Spittal Mill, Spittal, Treffgarne R.S.O Pembs" (I think RSO refers to a railway stop?)
Kelly's Directory Monmouthshire + S Wales Pt 1 1895 "Davies, Lewis, Spittal Mill, Steynton"
Slater's Directory of N + S Wales 1880 Pt 1 "Llewellyn, Thomas, Spittal Mill, Haverfordwest".
Worrall's Directory of S Wales 1875 "Llewellin, Thomas, Spittal Mill, Haverfordwest"
regards