I got these out of a junk yard off a 521 years back knowing I'd eventually need them and I don't remember if they go between the race and seal or between the race and hub. They are the 521 inner nla 32kb02 nsk brand. I've seen them with the rings installed and without. I'm currently changing one and was not going to use the black ring as the one that came out doesn't have one but was just wondering about them.
You may have to take a rat tail file or drill bit and tailor the carb holes to the adapter. Make sure you use lock tight on the two recessed bolts that go into the adapter to the intake or they will come loose in a few months.
On more than one engine not remembering what intake they were being l16 or l20b I've had to hog out the holes on the carb towards each other to get the carb to slide down on the studs. These were factory direct carb kits from weber that were supposed to be plug and play. I never took pics but reading around years ago told me most people never had this problem and didn't know what I was talking about.
Now that I think about it once was on a friends l18 once was my l20b and once was my l16. I never took pics of this problem.