Ok, I think I got it. Correct me if I'm wrong.
Screw this thing into the carb. Take the brass tubes off of my sync gauges and instead the rubber tube from the guages just slides over the spigot on the adapter? Put the rubber cap on it when not in use? If it makes a difference, I have the cheap dial gauges with the brass fittings.
It would make my day and probably save me hours if this means I don't have to search for the #3 carb screw that I drop down between the carbs every single time I try to reinstall.