Yep, you've got it right.
This is the takeoff for the vacuum gauges when you balance the carbs; there's one on each cylinder.
The front one is the tricky one to get to, and so I'd suggest screwing a connector from a vacuum gauge kit in and leaving a bit of hose connected, with a blanking bung (some suggest a golf tee, I use a M6 bolt with a bit of sealant) in it and tucking it away under the tank.
This means that you can get to it easily without having to strip the tank and airbox off when you next balance the carbs.
You'll need to do it on the other one, too, but on mine this is already connected to the Scotoiler, so it's easy to disconnect that and connect a gauge.
But for now, all you need to do is insert a suitable screw or cap-head and your problem is solved.
(TIP: you can order a set of four M5 or M6 brass vacuum take-offs from (EDIT: Morgan Carbtune); they don't list them but if you e-mail them they'll let you have the code to order them. They're only a few pounds, best to get a set of four as then you'l be kitted up when your mate's IL4 needs doing. Same goes for vacuum gauge sets, a four pot set is best IMHO)
It's not falling off, it's an upgrade opportunity.
