Could be 6.
They do seem to use some weird rubber.
The triangle bit goes dry and grey and moldy quite quickly. I was just about to treat again with some tyre shine spray.
If it's just looking tired / mouldy / marked up - it can be brought back to life quite easily with a bit of 1000 grit scotchbrite & some rubbing compound. Then treat with gummi pfledge (or similar).