If you look really closely, they are using the cushioned mounting hardware (rather than Thackery washers). but on the other hand, there doesn't appear to be any gap between what might be an 0-ring carrier and the manifold or the carb.
On a whim, I thought I'd make a bit of a comparison (I really don't know how fair it is). Assuming 45DCOE, each throat would be 1.77" across but I believe the largest main venturi is 40mm so the real diameter is about 1.57". That's a lot smaller than 1.75" Strombergs but do the Strombergs have the same sort of ramp up to the jet that the SUs have? That would drastically reduce the open area of the carb throat.