Flat axes like this were made during the Early Bronze Age. This one has a curved blade and a square butt. It would have been mounted with a wooden handle. The axe is probably from Ireland. Selby bought it from an Edinburgh antique dealer in 1894.
Murray, J 2005 The William McDowall Selby Collection, The Transactions and Dumfriesshire and Galloway Natural History And Antiquarian Society. LXXIX, 147-172.