Not all of the models had that same plug over the air mix screw. There's a good picture of the underside of a Keihin carb on page 9 here at the Jenks Bolts web site. Or, like Erwin said you could go to the definitive Triumph carb site Pat's How-To pages

A positive attitude may not solve all your problems, but it will annoy enough people to make it worth the effort. Herm Albright (1876 - 1944)