Hey, Brian
I think they are rubber just like stock.
Just less expensive than a new one from Nissan.
If I understand the website, the standard (8000 series) is similar to stock.
The plus model, (9000 series) is rubber also but is built to stay together if it separates and therefore is rated to 10K and is SFI approved.
The website mentions something about special rubber or something.
Check out the link above.
Mike.