The Royal Canadian Mint
Constructed in 1905-1908 to house a branch of the British Royal Mint, this building was one of several designed in the late gothic style by the Department of Public Works in the first part of this century. The building combines the function of a mint (producing coins and medals) with that of a refinery for gold produced by Canadian mines. As a royal mint it also produced imperial gold sovereigns which were a basis of currency. The institution came under the jurisdiction of the Government of Canada in 1931 and was re-named the Royal Canadian Mint.
Plaque via Alan L. Brown's site Ontario Plaques. Full page here.