Yes, that is right. The alloy bell that holds the rotor/disc is thicker to push the wheel out further and stop the back of the wheel hitting the caliper. An in-built spacer.
In the opposite, my bells are quite thin, so I have to either use a 3mm spacer or get a bigger offset wheel - but I still have a full spigot to mount on.
It's just 2 different ways of fixing the problem of caliper clearance.
I think I know what you mean by the part, it is like a piece of tube that extends the spigot? That'll be fine, it's only to stop wheel wobble at high speed, the bolts once tight will do all the important work.