Hi Mathias,
Looks more like an injector ring to me. Certainly more prominent on the right hand model than the left one but from your photo you can just make out a faint circle in the same place as the model on the right. I'd suggest somewhere in the felting or tumbling process one model or casting part got more of a tumbling than the other.
However only my opinion, for what it's worth.
i agree with Steve...its a casting injector ring...cool find...something to look for on these and regular wheels too
Please use a web hosting site (like photobucket) to store pictures so you can post them here, using attachments makes it hard to view the pictures when you have to scroll to see them. Seeing comparisons of models is hard to see with attachments too.