Found in garden shed of Mr Harry Barratt [WWI Gallipoli] and donated by his grandson. In 1877 Swan island was designed to defend shipping in Port Phillip and Hobson's Bays. The scheme, involving massive expenditure, comprised a forward defence system of forts at Port Phillip heads dominated by Fort Queenscliff and South Channel Fort (artificial island), and a network of coastal batteries. Some development of fortification had occurred in the 1850s, but it was the impetus of the Jervois-Scratchley reports which led to major work being undertaken in the 1880s. The overall defence plan involved a combination of guns, mines (known as electric torpedo) and ships to defend the entrance to Port Phillip Bay. Swan Island Fort would be used to protect the minefields in the harbour, Naval observation countermine charge 500lbs wet G.C Standard mine from 1877 to 1905 when charge was removed and mines used as buoys. In 1914 mines were recharged and used for defensive mining until Armistice.Brass plaque inscribed with information on naval countermineNaval observation or countermine charge 500lbs wet G.C. Standard mine from 1877 to 1905 when charge was removed and mines used as buoys. In 1914 mines were recharged and used for defensive mining until the Armistice.sea mine, plaque, pre wwi, naval countermine, swan island