English: 05/06/2011 at Heskin Hall Steam Fair.

Launched in 1957 the Model Twenty One was the first of the Triumph unit construction twin cylinder motorcycles from the Meriden factory. With a capacity of 349 cc, the engine was a classic Edward Turner design with a 360 degree crankshaft, twin gear driven camshafts and twin plunger oil pump. The cylinder block was cast iron with an alloy head and separate rocker boxes. The engine was dry sump lubrication and there was a single Amal Monobloc carburettor. Drive was by a conventional four speed gearbox, with a right foot gear lever.

There are two explanations for the 'Twenty One' name spelt out in full on the side panels. The first explanation is that it celebrates the twenty-first anniversary of Triumph Engineering Co Ltd. The second explanation is that the important US export market rated motorcycle engine displacement in cubic inches and 21 cubic inches equates to 350 cc.