That's a Sam Browne belt, Scaramouche.

The blazer was originally a boating club jacket made of garishly striped material. It was essential recreational wear at Oxford and Cambridge, whence it spread to the British idling classes during the 1860s, 70s and 80s, when rowing was a popular pastime and competitve sport. The blazer evolved into today's dark-blue sports jacket (with or without embroidered club badge on the breast pocket) during the reign of Edwardian VII, when Britain and Germany were engaged in a frantic naval arms race and amateur yachtsmen caught the tide of fashion and started styling themselves after the Royal Navy.

The three buttons on the cuff thing alludes to the manners and discipline of midshipmen in the British and Commonwealth navies, but that's mere folk etymology; the buttons actually indicated a warrant officer in the Royal Navy -- that is, a sailing master or senior artisan. This group included midshipmen qualified as master's mates.