Yes that is the 'Vent Lever Stop Screw' . I found a very good web site, www.buckeyetriumphs.org
a couple of years back when I rebuilt my Strombergs. It has everything you could wish to know about these carbs, a lot more than in the manual!
Here's the section relating to the stop screw, " The manuals I have show no spec for setting the vent lever adjusting screw. the two new carbs I have that I know nobody has screwed with have the lever adjusted so that it is vertical when the throttle is closed. Next , I inserted a suitable hose into the vent opening with the brass sleeve on the air filter side of the carb. I then sucked on the hose and noted that as the throttle is moved off idle, the vent is opened. I adjusted the stop screw such the vent is just sealed when the throttle is returned to idle I then adjusted it a further half turn to make sure it is sealed in the idle position".
These vents are part of the 'FED' spec set up which was not required in the UK but I guess being in the US you will still have this system, carbon canister, etc fitted? Even if not they are still a part of the carbs and can cause problems.
Most people will say Strombergs are a load of chew, which they are and that you should bin them in favour of SU's. As I'm intending to fit EFi at some point, I didn't want to do this. Some people in the States have actually converted these carbs into throttle body's, I guess so they look like standard carbs and can sneak through the US version of the MOT?