Thanks so much for the input!
- It is rubber.
- Agreed, the caliper was only to provide a general size reference; without it, it's impossible to tell if the seal is tiny or massive, so I figured I'd throw it in. I took a more careful measurement, and it comes in at around 11 mm in OD. But this is after it split - I can easily see it being 10 when intact.
- I took a more careful look at the fiche in the link I provided. According to that, the clutch rod seal at the back of the piston (#24) should be 18 mm in OD, which this one most definitely isn't.
- The chain o-ring suggestion was an excellent one. Both flat faces are grooved, as is the outer curved surface. I took a look at the o-rings of the master link of my brand new chain and they look exactly like this. I then took a look at the old chain - it's missing o-rings everywhere! The bike was massively neglected by the last owner and the old chain looked awful, but still, I'm really surprised to see that.
Looks like I've found the answer - thanks to this awesome community here.