I think it's the 1933 Triumph 6/1 wich had 650 cc, 25 bhp and was the first vertical twin.
It was a failure but other models followed later and this type of engine became the trademark of British bikes until the 70s.
In 1933, Triumph's chief designer was Val Page. Page's 'flagship' model - and his last great effort for Triumph - was the 6/1- 25bhp, four-speed, four-stroke 650 vertical twin, designed primarily for the sidecar market. It had a 360-degree crankshaft. The 6/1 soon won speed awards, lapping in tests at Brooklands at 100 mph. However exciting the concept, it proved a commercial failure in a conservative market. Other companies' twins at this time were all V-twins, so the vertical/parallel twin was an oddity. Triumph struggled along for a few more years, concentrating mostly on the booming car industry, but in 1936 it split the car division into a separate company. Ariel founder Jack Sangster bought the motorcycle division and appointed a new designer, the young Edward Turner, who had developed the Ariel Square Four. Val Page left Triumph to work for BSA.